remove obsolete email module

This email module was used in @nilclass's email app which was never
officially released. It doesn't seem to be maintained anymore.
Also, in https://github.com/remotestorage/modules/pull/34#issuecomment-42955620
@silverbucket commented that the messages module, that he added
in January, replaces it.

This fixes #41 (forgot to put that in the commit subject)