Add roles support to PostgreSQL user resource

This adds functionality to allow role attributes to be set for a PostgreSQL user. Example usage:

```ruby
postgresql_database_user "nomad" do
    connection postgres_connection
    password "keepitclassy"
    roles :superuser => true, :createdb => true
    action :create
end 
```

Let me know if anything needs to be cleaned up. I found this to be extremely useful so I hope it gets merged in!