Change Admin::getClass logic to use subclass/subject class

Fix Issue #1466 create/update routing problem in form creation, when id is 0
Fix in part Issue sonata-project/SonataDoctrineORMAdminBundle#250
Fix Issue #924 Broken field description when using subclasses
Merge pull request #1509 from tashanta/fixSubclass

Fix issue  sonata-project/SonataDoctrineORMAdminBundle#250