Use an AMD/commonjs/global module pattern for Luv.js

Uses [grunt-umd](https://github.com/umdjs/umd) to wrap the library up as a module on build.

There were 3 approaches I considered:
* Wrap at compile time.
* Create `pre.js` and `post.js` and add to concat.
* Convert each individual library into individual modules.

I went with wrapping at compile time because it was so easy. It just doesn't fit with the rest of the library because it isn't commented or show up in docco.

As you'll notice I only had to make a 1 line change to actual code to prevent it from assigning to `window.Luv`. The rest was adding `grunt-umd` to the dependencies and running it after concatenation of files.