[WIP] Support Doctrine MongoDB ODM

[alcaeus]

Fixes #26.

This PR introduces compatibility with MongoDB ODM. A quick summary:

• All Entity* classes have been replaced by corresponding Object* classes. To avoid BC breaks, existing classes remain but throw a silenced deprecation warning so people can update their code if they extend those classes
• ObjectManagerGetRepositoryDynamicReturnTypeExtension takes a managerClass constructor argument to be able to return different repository class names for ODM and ORM. The service configuration was updated to contain all three mappings for doctrine/persistence, doctrine/orm and doctrine/mongodb-odm.
• BC break: The doctrine.repositoryClass parameter was renamed to doctrine.orm.repositoryClass to better differentiate it from the other class names. This can be reversed, although I'm not sure of a good way to throw a deprecation notice for this.

Todos:

• Update tests to cover MongoDB ODM as well
• Document functionality
• Add type extension for Doctrine\Common\Persistence\ManagerRegistry

[alterphp]

@alcaeus May we help you on this ?

[alcaeus]

I've put work on hold since there are other PRs achieving the same thing and I haven't received any feedback from @ondrejmirtes on how to proceed. I'm not working on this until those questions are resolved to avoid doing work for nothing.