Update the getImportedExportedKeys() to support the foundationDB I_S.
This version supports both the referential constraints and the grouping
constraints. 
Add test for grouping constratints. Hold off on enabling the other FK
tests until all of the FK support comes online.
merge tjoneslo: Update the getImportedExportedKeys() to support the foundationDB I_S.

Merge pull request #15 from tjoneslo/pgjdbc/imported-keys
https://github.com/FoundationDB/sql-layer-jdbc/pull/15