New class AttributeHolders in perun-beans.

It is class for methods with couple of entities. It contains two attributes (primary and secondary) representing entities and basic methods.
New classes AttributeCacheManagerImpl and AttributeCacheManagerImplApi for methods of cache.

Copy methods from class AttributesManagerImpl to AttributeCacheManagerImpl. Generalization methods - use AttributeHolders instead of User in methods.
Modification of class AttributeHolders for holders which have attribute secondary null.

Object of AttributeHolders has couple of attributes primary and secondary. But secondary can be null for separate entities, therefore some methods have to be modify.
Writing Javadoc to interface AttributeCacheManagerImplApi for methods of cache.