Converted ColumnType to an interface and made 'core' module compile/work
Adapted other modules to work with the ColumnType-as-an-interface
change.
Minor fix to MongoDbDataContext
Merge branch 'master' into columntype_as_interface

Conflicts:
	couchdb/src/main/java/org/apache/metamodel/couchdb/CouchDbDataContext.java
Minor post-merge issue resolutions.
METAMODEL-54: Converted ColumnType from enum to interface