Database rake tasks take into account "username".

Hello Daniel,

please accept my request to your gem.

The problem, I've encountered: database rake tasks don't take into account "username" (PGUSER) for a DB user. They do handle PGHOST, PGPORT and even PGPASSWORD, but for some reason ignore PGUSER.

My usecase: I run "rake db:test:prepare" and see <<psql: FATAL:  password authentication failed for user "pavel">>