New method getMemberGroupsByAttribute()
 - getMemberGroupsByAttribute in entry check that attribute has group entity and session has right to read that attribute
 - removed unused imports from membersManagerBlImpl
Merge branch 'getMemberGroupByAttribute' of https://github.com/papperwing/perun into kubaTask02-getMemberGroupsByAttribute
Fix getMemberGroupsByAttribute method

- add better authorization
- better javadoc text
- better testing of entities in entry
- add test for this method
- return Utils and Roles to import in MembersManagerBlImpl (not the same
	task)
Merge pull request #329 from stavamichal/kubaTask02-getMemberGroupsByAttribute

New method getMemberGroupsByAttribute()