[WIP] Webhooks

Adds support for web callbacks in Iodized.  See: Issue #22 

TO DO:

- [x] Add persistence for webhooks
- [x] Add PUT, DELETE, GET support for webhooks
- [x] Dry up install task
- [ ] Dry up web handlers
- [x] Perform callback when an feature is added, modified or removed.

Mandatory gif:   
![gif](http://gifs.gifbin.com/092014/1411408222_pomeranian_vs_owner_eating_watermelon.gif)

/cc @madlep 



we should be running this async, probably via `Task.start/1` so it doesn't block changing the feature state