Implement backends

[martindevans]

Backends should allow Bastet to proxy HTTP requests to anything. The actual proxying logic for a given system is defined in a backend and every device is associated with a backend.

This allows bastet to become a generalized proxy for home automation, not just for CoAP (which now just becomes one more backend).

Work on backends is in https://github.com/bastet/Bastet/tree/Feature/Backends branch

[martindevans]

• Base HTTP backend (more or less just passthrough)
  • Hive backend (using the hive ReST API)
    • https://www.npmjs.com/package/bg-hive-api
    • https://github.com/csburton/hive_control
• CoAP backend (translate CoAP<->HTTP)