REST Hooks

@jathanism and I talked in #nsot and dropbox/pynsot#55 about this.

This is really important to have for me and brings Trigger + source of truth integration so much closer (better, at least).

Endless possibilities here, but for a Trigger use-case keeping a NetDevices loader up to date would be easy and fast. Could let Trigger do what it does best at loading via JSON, YAML, Mongo, SQLite, etc and just run a small service to catch the updates from NSoT.

References:
- http://resthooks.org/
- https://github.com/zapier/django-rest-hooks

Need to decide on what events we want to create. Obvious ones (I think) would be:
- Site
- Attribute
- Network
- Device