Support PostgreSQL unix domain socket paths

[ratson]

Support PostgreSQL URL using unix domain socket paths, e.g. postgres:////var/run/postgresql/db

[coveralls]

Coverage increased (+0.06%) to 88.491% when pulling 7e22a9b on ratson:postgres-socket into 628ed38 on joke2k:develop.

[hammadzz]

@ratson need to also support postgis which is the same as postgres but for GIS support on Postgres

[xrmx]

FYI for the default path this works fine psql:///db

[jefftriplett]

Can this test be manually re-run? It looks like AppVeyor might have bombed out for reasons outside of the tests if I'm reading this right.

I'm happy to volunteer/work on anything if there are concerns.