[Doctrine-Bridge] add a base compiler pass class to register doctrine mappings

| Q             | A
| ------------- | ---
| Bug fix?      |  no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | not on code, but defining best practices
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/2507

Reusable bundles providing model classes should not rely on the automapping provided by the doctrine bundles. What is more, the same model can often be mapped to Doctrine ORM, MongoODM, CouchODM and PHPCR-ODM.

This pull request adds a base class for a compiler pass that the concrete doctrine bundles extend to provide a default compiler pass that makes it trivial for bundles to register their mappings. See https://github.com/doctrine/DoctrineBundle/pull/177

The FOSUserBundle shows how this would look in practice. See https://github.com/FriendsOfSymfony/FOSUserBundle/pull/1081

I will create the documentation pull request as well as pull requests for the mongo, couch and phpcr bundles once we agree how exactly to do this.