PostgreSQL support

[jessepeterson]

A discussion topic about adding PostgreSQL support as a backend. A couple things I'd want to make sure is considered:

• Avoiding SQL anti-patterns (like select-before-insert/update race conditions vs. "upsert" techniques)
• Attempting to keep things in normal form (though, not a hard requirement for myself, more of a guide)
• Supporting multi-queueing and multi-pushing (like the existing MySQL backend)

[jessepeterson]

FYI PostgreSQL support was merged under #51. Thanks @sheshenia!