Expose the original function defining a module

This lets other browserify modules rebuild the bundle (if desired), which is helpful when creating with web workers (specifically, it enables the following kind of module):

```javascript
var worker = require('browserify-worker')

var thread = worker(function() {
  var EE = require('events').EventEmitter

  onmessage = function(ev) {
    postMessage('hello!') 
  }
}, __dirname)

thread.onmessage = function(ev) {
  console.log('got', ev)
}

thread.postMessage('you there?')
```