postgresql: add helper to allow ensuring no identity + validity period overlap

[brki]

Create an exclusion constraint for version_start_date and version_end_date for objects with the same identity. ( see http://www.postgresql.org/docs/9.3/static/rangetypes.html#RANGETYPES-CONSTRAINT).

This will only be usable if the btree_gist postgresql extension is installed (this requires superuser permissions to install).