Reorganize notifiers

This commit moves around the notifiers into a subpackage - graphitepager.notifiers - and simplifies the notifier attachment code. All notifiers now initialize themselves with their own client and also have an `enabled` flag that can be used to tell whether or not a notifier should be in use.

This commit makes all notifiers optional - though you really should use at least one notifier!
Merge pull request #23 from philipcristiano/reorganize-notifiers

Reorganize notifiers