Ignore messages from self

This ensures messages from ourselves don't end up trickling back into
callback_message, triggering a potential loop. This code was already
introduced previously in 4053eb6c242ef1c99e10257e099351e633202da7, but
was lost when the xmpp back-end was refactored extensively during the
move from xmmppy to sleekxmpp (the commit which actually removed it was
9a05e7f6609c749dfbeeae89df68d15bf2e4532b).

Fixes #247