node/Gruntification + new Plugin for Clicktracking

Hi!

So this pull request is a bit large but I hope you'll like the contents of it. 

So whats in it:
 - `plugins/clicks.js` : add a click-tracking plugin that sends a beacon everytime a an element on the page has been clicked. This includes viewport sizes and resolution of the browser window information aswell as the exact location of the clicked element. A use-case would be to create heatmaps for where a user has clicked or what the hot-spots of the current page are.

 - `package.json`: give boomerang the abillity to be deployed and built using a node/Grunt based toolchain. This way builds using uglify can be done with ease

 - `Gruntfile.js`: a better make in conjunction with the `package.json` file to 1) build boomerang.js as desired

 - `plugins.json`: exterenalize the way plugins get baked into the boomerang.js release

 - Tell the beacon server the currently built-in plugins: this should help discern beacon information and allow later analysis to better grasp what has been beaconed. Also if you are not in-charge of the boomerang.min.js build (ie. your customer has instead built his own boomerang.min.js with plugins) it makes it easier for the beacon reciever service to discern what data is actually viable to keep and explore

I hope to use this and other further planned improvements on the general codebase of boomerang and it's documentation to use it in my own projects. I would be glad if you could review this PR and tell me whats good, what could be better and where I missed something. 

Thanks in advance!

Andreas Marschke.