Allow load generators to register with Treadmill website

[bklang]

It would be nice to be able to run Treadmill web in something like Heroku, and have the queue workers elsewhere. In this way, workers can be placed where appropriate (behind the firewall) and allow execution of tests from a single control point.

• Allow workers to register with website
• Allow selection of a worker when creating a load test
• Heartbeat/mechanism to detect disconnected workers