Two new signals, plus documentation for the new signals and some previously existing undocumented ones

I've added two new signals called pre_email_confirmed and pre_email_confirmation_sent. They are sent shortly before the email is set to be confirmed on the db, and shortly before the email confirmation is sent out.  I'm actually not sure if those signals are only useful for our specific case or also for somebody else, we needed them because we wanted to make sure that the db updates for email_confirmed and email_confirmation_sent are not performed if the signal_handler raises an exception.

In the README.rst I have added documentation for these two new signals, plus for two which already existed but were not documented in the README.rst: email_confirmed and email_confirmation_sent