[DDC-3205] Metadata info

This PR adds support for showing the metadata for a single entity to the `orm:info` command.

- The output is formatted using the `TableHelper` if it is available (Symfony 2.4+). If the TableHelper is not available
it will format simply with `field: value`.
- Partial regex matches are accepted, e.g. `mapping:info Attraction`, `mapping:info Attraction.*`

E.g.:

````bash
$ php app/console doctrine:mapping:info "Sulu\Bundle\ContactBundle\Entity\Count"
+-------------------------------+-----------------------------------------------------------------------------------------------+
| Field                         | Value                                                                                         |
+-------------------------------+-----------------------------------------------------------------------------------------------+
| Name                          | Sulu\Bundle\TagBundle\Entity\Tag                                                              |
| Root entity name              | Sulu\Bundle\TagBundle\Entity\Tag                                                              |
| Custom generator definition   | None                                                                                          |
| Custom repository class       | Sulu\Bundle\TagBundle\Entity\TagRepository                                                    |
| Mapped super class?           | Empty                                                                                         |
| Embedded class?               | Empty                                                                                         |
| Parent classes                | Empty                                                                                         |
| Sub classes                   | Empty                                                                                         |
| Embedded classes              | Empty                                                                                         |
| Named queries                 | Empty                                                                                         |
| Named native queries          | Empty                                                                                         |
| SQL result set mappings       | Empty                                                                                         |
| Identifier                    | ["id"]                                                                                        |
| Inheritance type              | 1                                                                                             |
| Discriminator column          | None                                                                                          |
| Discriminator value           | None                                                                                          |
| Discriminator map             | Empty                                                                                         |
| Generator type                | 4                                                                                             |
| Table                         | {"name":"ta_tags"}                                                                            |
| Composite identifier?         | Empty                                                                                         |
| Foreign identifier?           | Empty                                                                                         |
| Sequence generator definition | None                                                                                          |
| Table generator definition    | None                                                                                          |
| Change tracking policy        | 1                                                                                             |
| Versioned?                    | None                                                                                          |
| Version field                 | None                                                                                          |
| Read only?                    | Empty                                                                                         |
| Entity listeners              | Empty                                                                                         |
| Association mappings:         |                                                                                               |
|   creator                     |                                                                                               |
|     fieldName                 | creator                                                                                       |
|     targetEntity              | Sulu\Bundle\SecurityBundle\Entity\User                                                        |
|     joinColumns               | [{"name":"idUsersCreator","referencedColumnName":"id","nullable":true,"onDelete":"SET NULL"}] |
|     type                      | 2                                                                                             |
|     mappedBy                  | Null                                                                                          |
|     inversedBy                | Null                                                                                          |
|     isOwningSide              | 1                                                                                             |
|     sourceEntity              | Sulu\Bundle\TagBundle\Entity\Tag                                                              |
|     fetch                     | 2                                                                                             |
|     cascade                   | Empty                                                                                         |
|     isCascadeRemove           | Empty                                                                                         |
|     isCascadePersist          | Empty                                                                                         |
|     isCascadeRefresh          | Empty                                                                                         |
|     isCascadeMerge            | Empty                                                                                         |
|     isCascadeDetach           | Empty                                                                                         |
|     sourceToTargetKeyColumns  | {"idUsersCreator":"id"}                                                                       |
|     joinColumnFieldNames      | {"idUsersCreator":"idUsersCreator"}                                                           |
|     targetToSourceKeyColumns  | {"id":"idUsersCreator"}                                                                       |
|     orphanRemoval             | Empty                                                                                         |
| Field mappings:               |                                                                                               |
|   name                        |                                                                                               |
|     fieldName                 | name                                                                                          |
|     type                      | string                                                                                        |
|     columnName                | name                                                                                          |
|     unique                    | 1                                                                                             |
|   created                     |                                                                                               |
|     fieldName                 | created                                                                                       |
|     type                      | datetime                                                                                      |
|     columnName                | created                                                                                       |
+-------------------------------+-----------------------------------------------------------------------------------------------+

Is there a better way to determine if a class mapping exists?