support `false` as a browser field value, added examples

`false` is really nice as a browser field value because it can serve as a handy ignore mechanism.

For instance for some code [like this](https://github.com/harthur/kittydar/blob/master/kittydar.js#L7):

``` js
if (typeof window === 'undefined') {
    var Canvas = require('canvas');
}
else {
    // browser already has Canvas defined
}
```

you can just `"canvas": false` in your package.json browser field:

``` json
{
  "name": "skip",
  "version": "0.0.0",
  "browser": {
    "canvas": false
  }
}
```