Livedev multibrowser

This is an alternative approach to #3 for including livedev2 in Brackets core as an experimental implementation as it was discussed in njx/brackets-livedev2#24.

 * keeps current ```LiveDevelopment``` untouched -> dependent extensions do not have to be modified
 * includes the entire ```livedev2``` implementation under ```/LiveDevelopment/MultiBrowserImpl``` folder which is probably not the more fancy way (under a second level) but allows integration be less intrusive.
 * move ```livedev2/LiveDevelopment.js``` out to ```LiveDevelopment``` root folder and renamed it to ```LiveDevMultiBrowser.js```
 * deleted ```livedev2/main.js```  
 * add quick modification to ```main.js``` to allow switching implementations:
   * there is a new config key added which allows enabling multibrowser impl (we can change that to be a preference if needed)
   * (not implemented yet) need to align some states to make the UI status update work 

@busykai, please review it. Thanks!