Change API to expose non-default pg version support via main type

Prior to these commits, you'd need to declare "postgresql::params" explicitly in your manifests in order to override the default postgres version for your distro.  Hunter pointed out that it was very inconsistent with other existing modules to make users declare "params" themselves.  These commits change the API such that you now specify those paramaters via the "postgresql" type (init.pp).

The init.pp previously managed the postgresql client packages.  Those have now been moved into a ::client type.