Added task/target 'dev' to speed up rebuild via 'grunt watch:dev' for live debugging.

When having to debug and adjust for drupal.org UI changes, `dreditor.js` should rebuild as quickly as possible.  As I'm developing on Windows, any unnecessary file operation slows down the `grunt watch` process in substantial ways.

This PR introduces a new grunt task/target `dev`, which only rebuilds/recompiles the `dreditor.js` script itself, so that Firefox/GreaseMonkey can consume it immediately.