add bower.json

Hi - I wonder if you'd consider adding a bower.json file? Doing so means that bower/AMD users can `bower install jshint` and do this...

```js
var JSHINT = require( 'jshint' );
```

...instead of this...

```js
var JSHINT = require( 'bower_components/jshint/dist/jshint' );
```

...(or setting up a custom paths config) because Bower looks for the `main` property inside a `bower.json` file.

Thanks!