Add JavaScript bundling (concatenation, minification)

Hi,

this PR adds support for
- concatenating multiple `.js` files into one resource
- minifying the concatenated scripts by using the [Google Closure Compiler](http://closure-compiler.appspot.com/) (by using its REST API)
- wrapping the concatenated scripts into an IIFE/IFFY/SIAF

I'm not sure if this is somehow against the lightweight nature of Martini - but hey, this is contrib :)

I'm open for suggestions, improvements and test cases.