Support 'postgres' protocol in heroku like db urls.

Heroku like db urls were supported with 'postgresql' protocol.
In my heroku app, the DATABASE_URL was set with 'postgres' protocol,
so that the db url could not be parsed.

This change supports both 'postgresql' and 'postgres' protocols.
Merge pull request #33 from magro/support-postgres-protocol

Support 'postgres' protocol in heroku like db urls.