Update JS API after changes in Crosswalk
Update JS API after changes in Crosswalk

Use extension.postMessage/setMessageListener instead of Crosswalk
variants. Use 'exports' object instead of defining 'brackets'
ourselves. Crosswalk plug contents of exports into 'brackets' by
itself.

crosswalk-project/brackets-crosswalk,crosswalk-project/brackets-crosswalk,crosswalk-project/brackets-crosswalk,crosswalk-project/brackets-crosswalk