Improve grunt build tasks and add grunt dist

[chill117]

Change the build tasks to be more intuitive:

• build:plugins will build only plugin files
• build:joint will build the joint core files (and plugins)
• build:bundles will build webpack and browserify bundles
• build:all will run all build tasks
• build will run build:joint
• dist will run all build tasks then re-build the dist directory

Tests and demos should link to the build files (not the dist files).