Add support for PostgreSQL

[jkutner]

A few concerns:

• Search triggers and table are not included in postgresql schema. I couldn't get search working, but I think that's because Lucene.
• Environment variables now override default and config.clj for uberjar (couldn't figure out how to set PG URL without hard coding otherwise, but this is generally good i think).
• Depending on how the JDBC URL is set, the file clojars-postgres.sql or clojars-postgresql.sql might be used. I'll fix this in a future PR.

[tobias]

Thanks for taking a look at this - I hope to have a chance to review and merge it soon.

[tobias]

Thanks again for this @jkutner - I know you did it years ago, but I used it as a starting point for finally moving us to postgres in #736.