Add Support for Pushbullet.com Notifications

Pushbullet is similar to Pushover.  It allows you to send notification messages directly from MH to your devices.

A device can be an iOS or Android phone, a windows PC, or any Chrome browser.  All of the clients are free and the service is free.

Pushbullet has many features:
- Send notes, urls, files, lists, and addresses
- Add multiple devices and friends
- Choose which devices and friends to send a particular push to

Right now the code provides a user friendly mechanism to send notes.  A low level module also provides raw support for sending whatever you want to the Pushbullet API.  In future commits, I hope to make additional features easy to use.

The addition of this feature to MH and some of the code was inspired by George Clark's work on the Pushover support.
Merge pull request #422 from krkeegan/pushbullet

Add Support for Pushbullet.com Notifications