Allow to specify privileged user for database drop/create (PostgreSQL only)

[sauliusgrigaitis]

I'm surprised why so much people use privileged user for database access from Rails. App database user should not be allowed to drop/create databases. So I added options to specify privileged username and password (PostgreSQL only) for those who use unprivileged user for Rails app:

set :db_remote_superuser, 'postgres'
set :db_remote_superuser_password, 'secret'
set :db_local_superuser, 'postgres'
set :db_local_superuser_password, 'secret'

Solves #40

[numbata]

What about idea of PR #100 : allow to use local database.yml with custom credentials for remote database?