making module a bit more reusable.

Just some small changes to make the module usable without having to fork it. I'm a puppet newbie so if there is a better way of doing this...

    node 'mynode' extends default {
        class { 'postgresql': }  # also import stdlib
        class { 'nginx::params': # want to override a setting
            nx_client_max_body_size => '5G';
        }
        class { 'nginx': }
    }