Support AltBeacons

Working towards #70. Feedback appreciated.

Issues:
- [ ] The beacon message is a different than the existing iBeacon message. The main difference is that distance is given in meters as a double rather than Near/Far/etc. I've also included some additional BT info - beacon manufacturer, etc
- [ ] No rate limiting - when a beacon is found, it publishes a MQTT message for every BLE report (~1Hz). Should be modified to push a report when beacon first found, maybe every meter of change (?), and when beacon is no longer detected
- [ ] Beacon message class not idiomatic, is hastily thrown together
- [ ] Untested for reliability; I'm trusting the AltBeacon library to do its thing (and trusting that I'm working with it perfectly, which is unlikely)
- [ ] No battery saving mode - to do this, I think I need to get at the Application.context from my Beacon ProxyService. Haven't looked into this very deeply, may be mistaken.
- [ ] Commits have a bunch of misc unnecessary project changes