E-mail notification support.

Notification triggers are limited to system events for
now, so new notification types will require new
system event definitions (such as the recently-added
KEG_VOLUME_LOW event).

Notification backends are abstracted (see
notification/backends/base.py) since non-email notification
channels seem plausible (SMS, web service, etc). Time
will tell whether the extra abstraction is useful,
of course..

Closes #144 & #148.