v.164 has ISSUE-18 resolution related changes

I have updated the R2RML draft based on resolution for ISSUE-18 (reuse 
of SQL query, or logical table, in multiple TriplesMap elements).

However, to avoid complexity involving class hierarchy, I have not 
introduced any subclasses for the new rr:LogicalTable class.
If needed, we can discuss this tweak in the next meeting.

Thanks,
- Souri.