check if schema exists in backwards compatible way

The "IF NOT EXISTS" clause (from UR::DataSource::RDBMS) is not supported until
PostgreSQL 9.3 so instead of using that we override the behavior to query the
information_schema instead.
Merge pull request #47 from nnutter/pg

check if schema exists in backwards compatible way
Remove commented-out breakpoint.
Whitespace.
Die on bad properties.

Otherwise the lister proceeds with the bad rule omitted.  This may list
the entire table, which is probably not what is desired.
Check the front of dot-properties for validity as well.
Allow callers to handle @extra if they wish.
Croak instead of dying.
Remove extraneous 'g' modifier.
Merge pull request #48 from tmooney/verify_lister_properties

Better Error Handling for Listers