(#23): Add unique field into LBEAttributeInstance table.
Create unique check function
(#23): Implement feature for unique values.
Merge branch 'unique' into v2

Conflicts:
	dao/MongoDao.py
	directory/views.py
(#29): Add '-' button to remove multivalue field.
(#25): using asynchronous feature instead of current one about the DN attribute replacing to the Target Server.