css

This adds CSS live document support on top of sebaslv's PR#3.
It's actually redundant in this version, but it will be needed later!)  We have an implementation to keep the Live Preview in the same browser tab (it's in our internal review pipeline at the moment https://github.com/busykai/brackets-livedev2/pull/12).  Once it's integrated we won't be calling ```open()``` in ```_onDocumentChange()```.  I can remove it from here now, or just keep it here till we have the PR to merge the single tab implementation.