MDL-51582 grunt: task improvements

MDL-51582 grunt: task improvements

* Introduce the 'css' task - this will give us flexibility to do things
  in future rather than relying on the inbuilt 'less' task (e.g. sourcemap
  generating task which doesn't run by default)

* Introduce 'amd' task which runs only amd tasks

* Introduce 'js' task which runs all JS (amd/shifter) tasks

* Tweak the ordering so that slow shifter always runs last (useful for
  people like me who forget to specif the task)