fix(build): use grunt-newer for styles and jshint

Applied @tschaub's changes to generator-webapp using grunt-newer for jshint and
style copying.

See yeoman/generator-webapp#193

Author: passy

templates/common/Gruntfile.js

@@ -41,7 +41,7 @@ module.exports = function (grunt) {
       },<% } %>
       styles: {
         files: ['<%%= yeoman.app %>/styles/{,*/}*.css'],
-        tasks: ['copy:styles', 'autoprefixer']
+        tasks: ['newer:copy:styles', 'autoprefixer']
       },
       gruntfile: {
         files: ['Gruntfile.js']
@@ -410,7 +410,7 @@ module.exports = function (grunt) {
   ]);
 
   grunt.registerTask('default', [
-    'jshint',
+    'newer:jshint',
     'test',
     'build'
   ]);