Start integrating with app manager

Work in progress...

It mainly requires review on the platform side part before being taken seriously here,
but feedback much appreciated for both patches on how we are going to integrate with the simulator addon.

Here, we register to a firefox JSM that will be queried by the app manager code to know if there is some simulator being installed, and if yes, launch/close a new simulator:
https://github.com/ochameau/mozilla-central/commit/234071900839a5f1986e63cacbb2a3a8eead0554

Otherwise, on simulator side, I basically forked remote-simulator-client in order to only keep process code (and strip all DebuggerClient logic). I think it will be handy to land this code in the simulator addon to easily test the new firefox app manager. But later on, we should spin another addon, that do not ship the dashboard.

With this, we should be able to install multiple simulator addon, so there is some refinement to be defined, where we would have one new simulator addon id per Firefox OS big release/branch: 1.1, 1.2, ...