Add list of registered processes to erlport.app

This adds `{registered, []}` to *ebin/erlport.app*, which according to the [Erlang documentation](http://www.erlang.org/doc/design_principles/applications.html) is required.

The reason for me adding this is that [relx](https://github.com/erlware/relx) expects `registered` to be present and fails when it's not.