[OLINGO-168] Enhance Edm for Service Document serialization

Also implemented a first version of the service document serializer. This
has still to be moved into the commons core.
Some client-side refinements, generally Edm enhancements look fine
Merge branch 'olingo167'

Conflicts:
	lib/client-core/src/main/java/org/apache/olingo/client/core/edm/EdmEntityContainerImpl.java
Merge from master