Initial implementation of drag and drop to open files

The pull request adds an initial implementation of file association and drag and drop to open files.

**Implemented on the Mac:**

* Drag and drop files onto the Brackets icon in Finder or Dock
* Right-click and select Open With > Brackets.app

These work whether or not Brackets is already running

_Missing on Mac:_ drag and drop onto a Brackets window. Chromium is swallowing all drag events so they never make it to the native shell. 

**Implemented on Windows:**

* Drag and drop files onto the Brackets icon in Explorer. This will always launch a new instance of Brackets
* Drag and drop files onto the Brackets window

_Missing on Windows:_ Right click and select Open With > Brackets.exe. This requires registry modifications that would be best to do from the installer.

**NOTE:** There will be a corresponding brackets pull request that is required to open files at launch time.