Add @var annotations to the Traits for TYPO3.Flow support

The TYPO3.Flow Frameworks relies heavily on @var annotation in Entities
to properly handle them. This commit adds those @var annotations to
the DoctrineExtension Traits to be compatible with TYPO3.Flow
Merge pull request #1092 from mneuhaus/flow

Add @var annotations to the Traits for TYPO3.Flow support