Implemented :doctrine_em configuration option

* Added :doctrine_em symbol with default value of `false`
* Added `before` hooks for relevant symfony:doctrine:* commands to assemble --em flag
* Added flag variable to commands being run
* Added tests for --em flag in spec file

Fixes everzet/capifony#321
Merge pull request #322 from EvanK/symfony2-doctrine-em-flags

Added :doctrine_em configuration option for custom Doctrine entity manager