Added a bower.json file so this can be published in bower

It would be wonderful to be able to include protobuf.js in a project using [bower](http://bower.io/), which is a package manager for front-end components.

I've taken the liberty of packaging this repo up as a bower package so that it can be published.

The package can then be registered with the following command:

```bash
bower register protojs git@github.com:sirikata/protojs.git
```