Issue #38 Multiple entity managers support

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | NA
| Fixed tickets | #38
| License       | MIT

This PR is an attempt to solve #38 by allowing to specify a different migrations config per entity manager. This feature is targeted at people who use multiple databases for a single Symfony2 project.

As an example :

```yaml
doctrine_migrations:
    em1:
        dir_name: %kernel.root_dir%/DoctrineMigrations/Em1
        namespace: Application\Migrations\Em1
    em2:
        dir_name: %kernel.root_dir%/DoctrineMigrations/Em2
        namespace: Application\Migrations\Em2
```

Notes : 

*  This additional level of nesting in the config is totally optional, the standard way still works
* An exception is thrown in 2 cases : 
  * If the config file is defined on a per-entity manager basis but no --em option is provided 
  * If there is no config entry defined for the provided --em option

Thoughts ?