Tweaks to make this plugin easier to use with Browserify

With these tweaks in place, you can load up PXU photosets just by running something like this, no need for separate CSS or JS files:
```js
var $ = require('Extended-Tumblr-Photoset');
$('.photo-slideshow').pxuPhotoset();
```
There's also a crack at AMD support (I added a UMD-specified definition, which does AMD too), but that one doesn't do CSS because I'm not familiar enough with AMD to know how it handles CSS.

This is all fully backwards-compatible with using the script the "traditional" way, in a <script> tag.

(Note that this request's dependent on the earlier package.json pull request. Not sure what the right way to handle situations like this is?)