Add support for PostgreSQL system catalogs

[kyleconroy]

We currently don't have any support for the PostgreSQL system catalogs.

To list all of the catalogs, we can use the following pg_dump command:

pg_dump -s -t pg_*.*

This output will need to be cleaned up before it can be parsed, as it includes some complex views that might not be parsed correctly.

See #1170 for an example of how this breaks queries.