Add support for AMD and CommonJS.

Would be great to use with [Browserify](http://browserify.org/)/[RequireJS](http://requirejs.org/)/etc.

```js
var Webcam = require('webcam');
Webcam.attach('#webcam');
```
```js
require(['webcamjs'], function(Webcam) {
  Webcam.attach('#webcam');
});
```
```html
<script src="js/webcam.js"></script>
<script> Webcam.attach('#webcam'); </script>
```