grunt: remove superfluous check if dir exists
modularise scss files
grunt: add css core dir to sass includePaths
clean up js structure

- rename the generated base.js bundle to omni.js
- rename src/js/base.js to index.js, this is to match the upcoming convention
  for javascript in modules; any module that has a js/index.js file in
  the module directory will be included in the bundle
grunt: include javascript from modules

This script will now automatically loop all modules to see if they have
a js/index file. If so, it will add it to the main browersify bundle and
set up js watch targets.