Add background notification plugin (cross platform)

Works with ubuntu, osx notification center (using terminal-notifier), and cygwin.

Supports having a custom message hook (in case you need it to say 'holy smokes batman' on failure).

**Linux**
![screenshot from 2014-11-07 15 58 36](https://cloud.githubusercontent.com/assets/326829/4962187/256b465c-66da-11e4-927d-cc2fc105e31f.png)

**Windows**
![screenshot from 2014-11-07 15 55 00](https://cloud.githubusercontent.com/assets/326829/4962159/a2625ca0-66d9-11e4-9e91-c5834913190e.png)