add support for browserify [was: browserify build fails]

I'm trying to build mathjax-node to bundle with browserify.

main.js file:

```
var mjAPI = require('mathjax-node')
```

Regular node execution works fine

```
$ node main.js 
```

But browserify returns error

```
$ browserify main.js --node
Error: Cannot find module 'canvas' from 'path/mj/node_modules/jsdom/lib/jsdom/level2'
```

How to fix this?
