Improvements to Close Others

As mentioned in https://github.com/adobe/brackets/pull/6020#issuecomment-36387113, I just did some improvements to the Close Others extension:
* Removed the `DocumentCommandHandlers` module - it wasn't used
* Renamed modules (beginning with an uppercase char) and vars (camelCase) to match the Brackets standard
* Using the handler `beforeContextMenuOpen`

Unit tests still passing.
@JeffryBooher