Using SonataClassificationBundle without SonataMediaBundle

- Using an explicit variable rather than calling the function "interface_exists"
Merge branch 'master' of git://github.com/sonata-project/SonataClassificationBundle
Fatal error in CategoryManager

When making a call to getRootCategoriesPager or getSubCategoriesPager methods from the CategoryManager we obtain the following error:

ContextErrorException: Warning: Missing argument 1 for Doctrine\ORM\EntityRepository::createQueryBuilder()

The patch fixes this problem.
Revert "Using SonataClassificationBundle without SonataMediaBundle"

This reverts commit bf51e055b9764c3a984092add6d2984e3cdd993e.
Merge pull request #38 from zapoyok/categoryManager

Fatal error in CategoryManager