Add support for grunt watch

Here is my take on using `grunt watch` to to develop an extension.

This PR assumes that one develops by loading the extension against the /app directory. If developing/testing loading against the /app directory then the current implementation of `grunt watch` doesn't work.

All modifications are to `Gruntfile.js` template and the `readme.md`.

For Coffeescript support, the changes are very minimal. Simply have `coffee:dist` output to `app/scripts`. Additional js files can be added via the build:js in popup.html. The `grunt build` works as expected.

New SASS modifications pre-process to app/styles/main.css. In this way no additional css files need to be added to popup.html.

I added an option called `--with-sass` (to be used with the `build` task) because the `app/styles` directory needs to be excluded by cssmin because concatenation has already occurred by the `watch` task. The `.tmp/styles` directory does have the individual css files output from the `watch` (`compass:server`) task and are the proper target for cssmin.
For users who are creating pure css files in the `app/styles` directory we leave `grunt build` the way it is.