Documentation for the new PhpUnit bridge

[xabbuh]

symfony/symfony#13398 introduced the new PhpUnitBridge that will ship with Symfony 2.7. I think we should at least write a bit about it in the testing section of the cookbook.

[wouterj]

I'll try to take care of it (already started writing an article about preparing for sf3)

[stof]

This should be reopened. We should have a documentation page for the component itself, showing how to use it and describing its nice features (different modes, matching a single deprecation, etc...)