added REGISTER handler to reporters app
added REGISTER handler to reporters app

in nigeria, we were using the forms app to do this. i'm _trying_ to use
the forms app where i can, but this seems like something that the
reporters app should be able to do by itself.

dimagi/rapidsms-contrib-apps-dev,rapidsms/rapidsms-contrib-apps-dev,rapidsms/rapidsms-contrib-apps-dev,dimagi/rapidsms-contrib-apps-dev