Update custom ID generator to allow a custom mapping type

Currently, the implementation of custom ID generators (#402) hardcoded the type to custom_id. When your generator returns a MongoId instance, the value isn't converted which means it can't be used in areas like ```UnitOfWork#addToIdentityMap``` ([source](https://github.com/doctrine/mongodb-odm/blob/4a5457eb713b12d6124db9817c634533d8018044/lib/Doctrine/ODM/MongoDB/UnitOfWork.php#L1512)) .

It would be nice if you could specify a type when using a custom strategy. This would allow a user to specify a type of "object_id" when using a custom generator that returned a MongoId.

An example:
```
/**
 * @ODM\Id(
 *     strategy="custom",
 *     options={
 *         "class"="CustomObjectIdGenerator",
 *         "type"="object_id"
 *     }
 * )
 */
protected $id;
```