Gruntify boomerang and add package.json

 Replace most of the functionality in the Makefile with Grunt and npm.

 A new workflow to build boomerang would be:
 ```shell
 $> cd boomerang
 $> npm install
 ...
 $> grunt
 ...
 $> ls build/
 boomerang-0.9.0-debug.js
 boomerang-0.9.0-debug.min.js.map
 boomerang-0.9.0.min.js
 boomerang-0.9.0-debug.min.js
 boomerang-0.9.0.js
 boomerang-0.9.0.min.js.map
 $>
 ```
 This /should/ streamline the process of getting an initial build of
 boomerang.js as fast as possible for the user and removes friction
 towards getting up and running.

 As a side effect using the plugin.json file to configure the plugins
 baked into the final boomerang release, you get an easier to reproduce
 build that makes development of plugins easier.