Handle kwargs in postgres user creation

These commits add support for `createdb`, `createrole`, `superuser` and `encrypted` kwargs in postgres user creation (``fabtools.postgres.create_user()`` and ``fabtools.require.postgres.user()``)

This allow to give more right to user on creation like in the [postgres createuser app](http://www.postgresql.org/docs/9.1/static/app-createuser.html)