[sensors] start() to return a cancellable promise that resolves on first reading

tobie has just created a new issue for https://github.com/w3c/sensors:

== start() to return a cancellable promise that resolves on first 
reading ==
The `start()` method needs to return a promise that resolves on first 
reading. It's unclear right now whether `stop()` also needs to return 
a promise or can be sync (I feel it's the latter).

Depends on 
https://github.com/promises-aplus/cancellation-spec/issues/6.

Please view or discuss this issue at 
https://github.com/w3c/sensors/issues/94 using your GitHub account