Upgrade R2RML to xR2RML mapping document
Move R2RMLTable to xR2RMLTable and R2RMLSQLQuery to xR2RMLQuery.
Rename Constant.LogicalTableType.SQL_QUERY to more generic QUERY.
Rename logicalTable to logicalSource in R2RMLTriplesMap
Support use of xrr:reference as a regular rr:column