Double heartbeat

This implements a secondary heartbeat system which uses HTTP instead of WebSocket. It's backwards compatible with the current `Register` and `RegisterForever` methods.

Two additional new API call's are added, `RegisterHTTP` and `RegisterHTTPForever`. On kontrol we have a new `registerHTTP` method which is called only by these two methods. So existing kites will not affected by any means, however new kites can choose to use the new HTTP based heartbeat system. If everything goes ok, it will eventually replace the current Register/Heartbeat mechanism over WebSocket entirely.