Bug 1035277 - Add option to not restart b2g....

So with this change we can now have:

```js
var host = new Host({ restart: false });
host.start(profile, callback);  // Does not restart
host.start(profile, { restart: true }, callback);  // Does restart
host.start(profile, callback);  // Does restart since last call overwrote object state
```
Seems unintuitive... thoughts?