Comply with Circe guidelines for extension modules

The Circe wiki says that extension module foo should:
- be provided in a circe-foo.el file
- provide the circe-foo feature
- provide enable-circe-foo and disable-circe-foo [0]

Functions circe-notifications-enable and circe-notifications-disable
have been renamed to enable-circe-notifications and
disable-circe-notifications respectively.

[0] https://github.com/jorgenschaefer/circe/wiki/Hacking

Also be friendlier to OSX users who can't use dbus.