Bower installation

I've added a bower.json file to allow for better integration with the bower package manager.
This repo is already register with bower

`bower search shiv`

![Screen Shot 2013-04-28 at 18 33 56](https://f.cloud.github.com/assets/678372/436106/de8c0d2e-b029-11e2-9320-b9e711acd4e3.png)

As it has already been registered it can be installed using bower. But specify the "main" files in the bower config fixes some issues I'm having using bower and other grunt tools.
This should also make it easier for other user to install the html5 shiv too.

If you'd like I can also update the readme to document how to install this repo using bower.