Added notifier structure to abstract notifying different services.

Right now only the email notifier is being used. This is a first step
to allowing multiple notifiers and eventually: user defined notifiers.
Do not allow adding an invalid notifier.
Added new method to add labels to an issue/PR.
Created github notifier.

This notifier adds a “Stale” label to PRs that should be notified of
being stale.
Restructured email notifier to exist in the notifier directory.
Added option for github label notifier.

Email is now optional if the label option is specified.
Fixed the email test from move.

Includes change to grunt task.
Refactored email spec test to be more maintainable.

This is a better test as it ensures the email template is used properly.
It also makes it so any minor template changes (grammar/spelling) don’t
require an update to the test, so it becomes less brittle.
Merge pull request #4 from cjsaylor/notifier-structure-labels

Add notifier structure to support multiple kinds of notifiers.