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**.

Usage example:

~~~go
import (
  "github.com/codegangsta/martini"
  "github.com/codegangsta/martini-contrib/bundle"
)

func main() {
  m := martini.Classic()

  scriptBundle := bundle.NewScriptBundle(true, true)
  scriptBundle.AddFiles(
    "public/js/one.js",
    "public/js/two.js",
    "public/js/three.js",
  )

  m.Get("/js/app.js", scriptBundle.Handler())

  m.Run()
}
~~~