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.
Update this to the main repo URL on github.