EntityMapper for composite columns using OneToMany

New EntityMapper for composite columns which uses a OneToMany
relationship between the parent entity, which has the Id for the row
key and child entities for each column with the value being a field of
the child.
resolve #327 . Unit test is included.
Add 'native' query to EntityManagers.

Extend the functionality of CompositeEntityManager to query a column
slice using the new NativeQuery interface
Merge pull request #323 from elandau/new_composite_mapper

EntityMapper for composite columns using OneToMany
Merge pull request #331 from michaelp/master

Bug fix. Resolve #327