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)

Gruntfile.js

@@ -219,14 +219,11 @@ module.exports = function(grunt) {
             grunt.task.run('shifter');
         // Are we in an AMD directory?
         } else if (path.basename(cwd) == 'amd') {
-            grunt.task.run('jshint');
-            grunt.task.run('uglify');
+            grunt.task.run('amd');
         } else {
             // Run them all!.
-            grunt.task.run('shifter');
-            grunt.task.run('jshint');
-            grunt.task.run('uglify');
-            grunt.task.run('less');
+            grunt.task.run('css');
+            grunt.task.run('js');
         }
     };
 
@@ -236,8 +233,13 @@ module.exports = function(grunt) {
     grunt.loadNpmTasks('grunt-contrib-jshint');
     grunt.loadNpmTasks('grunt-contrib-less');
 
-    // Register the shifter task.
+    // Register JS tasks.
     grunt.registerTask('shifter', 'Run Shifter against the current directory', tasks.shifter);
+    grunt.registerTask('amd', ['jshint', 'uglify']);
+    grunt.registerTask('js', ['amd', 'shifter']);
+
+    // Register CSS taks.
+    grunt.registerTask('css', ['less:bootstrapbase']);
 
     // Register the startup task.
     grunt.registerTask('startup', 'Run the correct tasks for the current directory', tasks.startup);