Notifications via push

[groenroos]

Add a facility to send email notifications (issue #4) via push notification (i.e. out-of-the-box integration with a pret-a-porter push notification API service).

[groenroos]

Seems like this would be easier than expected with the node-pushnotifications library, for which Sapling could provide a thin wrapper.

The true challenge is figuring out a way for the developer to define notifications across email, push and SMS with minimal code. Likely some magical file structure that acts as the definition of what notifications are available and on what formats? Would need to take #24 into account as well.