Allow apache::mod to specify module id and path

Add $id, $path and $lib_path parameters to apache::mod. These parameters
are used in #342. The purpose of the whole thing is to plit #342 into
smaller parts to make review process easier, see
https://github.com/puppetlabs/puppetlabs-apache/pull/342#issuecomment-25423813

This PR covers all changes introduced by #271 to manifests/mod.pp. I
just don't have test/mod_load_params.pp as in #271.