Unified data types [actualType and subType]

This is the result of https://github.com/nelmio/NelmioApiDocBundle/issues/410.

This PR aims to provide a uniform way of declaring data-types of parameters for
parsers and handlers to follow. In turn, this would allow formatters to
determine data-types in a cleaner and less volatile manner. (See use-case that
can be improved with this PR:
https://github.com/nelmio/NelmioApiDocBundle/blob/master/Formatter/AbstractFormatter.php#L103)

This is possible by the addition two properties to each property item in
`response`, and `parameters` fields in each API endpoint produced by the
`ApiDocExtractor`:

* `actualType` Contains a value from one of the `DataTypes` class constants.

* `subType` Can contain either `null`, or any other `DataTypes` class constant.
This is relevant when the `actualType` is a `DataTypes::COLLECTION`, wherein
`subType` would specify the type of the collection items. It is also relevant
when `actualType` is a `DataTypes::MODEL`, wherein `subType` would contain an
identifier of the model (the FQCN or anything the parser would wish to specify)

Examples:

```php

array(
    'id' => array(
        'dataType' => 'integer',
        'actualType' => DataTypes::INTEGER,
        'subType' => null,
    ),
    'profile' => array(
        'dataType' => 'object (Profile)',
        'actualType' => DataTypes::MODEL,
        'subType' => 'Foo\Entity\Profile',
        'children' => array(
            'name' => array(
                'dataType' => 'string',
                'actualType' => DataTypes::STRING,
                'subType' => null,
             ),
            'birthDate' => array(
                'dataType' => 'date',
                'actualType' => DataTypes::DATE,
                'subType' => null,
            ),
        )
    ),
    'languages' => array(
        'dataType' => 'array of strings',
        'actualType' => DataTypes::COLLECTION,
        'subType' => DataTypes::STRING,
    ),
    'roles' => array(
        'dataType' => 'array of choices',
        'actualType' => DataTypes::COLLECTION,
        'subType' => DataTypes::ENUM,
    ),
    'groups' => array(
        'dataType' => 'array of objects (Group)',
        'actualType' => DataTypes::COLLECTION,
        'subType' => 'Foo\Entity\Group',
    ),
    'profileRevisions' => array(
         'dataType' => 'array of objects (Profile)',
         'actualType' => DataTypes::COLLECTION,
         'subType' => 'Foo\Entity\Profile',
    ),
    'address' => array(
        'dataType' => 'object (a_type_a_custom_JMS_serializer_handler_handles)',
        'actualType' => DataTypes::MODEL,
        'subType' => 'a_type_a_custom_JMS_serializer_handler_handles',
    ),
);
```

When a formatter omits the `dataType` property or leaves it blank, it is
inferred within `ApiDocExtractor` before everything is passed to formatters.
Merge pull request #413 from bezhermoso/unified_data_types