Added package.json for NPM

I added a package.json so the module can be installed as an NPM package with Browserify.

With this you can install the package with npm:

    $ npm install git+https://github.com/sverrejoh/javascript-detect-element-resize

And then use the page with require from your Browserify project:

    // Content of index.js
    require("javascript-detect-element-resize");

Build into one bundle containing everything with Browserify:

    browserify index.js > bundle.js

I didn't change the code at all, this is just metadata. 

I think it would be nice if the package had its own namespace, which exported  `addResizeListener` and `removeResizeListener`, so you could use the code like this (instead of putting it on window):

````
var detectResize = require("javascript-detect-element-resize");
detectResize.addResizeListener(element, func);
````

But this requires some change in your code, and build step, so I didn't want to start on it.

Using Browserify you could have the canonical version as a Browserify/CommonJS module, and then just from that generate a package that works standalone, or with AMD/Bower.

https://github.com/substack/browserify-handbook#standalone

As I said, a little bit of changes, but I'd love to help if you're interested :).