Don't use jspm to load Picturefill

Unfortunately jspm throws a parse error when trying to load the Picturefill polyfill. @OliverJAsh believes this to be a bug which I will be submitting to jspm. The module has a UMD definition at the bottom of the page, and the jspm cdn version thinks this to be the case, but this error is thrown:

```javascript
Uncaught AMD module github:scottjehl/picturefill@2.0.0/src/picturefill did not define 
```

Therefore, we are temporarily going to load Picturefill with an old'skool `<script>` tag until the issue has been resolved. 
I was wondering if the polyfill had to be loaded before the DOM was parsed, as that's quite a common requirement with polyfills (e.g. HTML5 Shiv). I think Picturefill does shiv the PICTURE element, so if we were using it we would have to shiv it ourselves (synchronously) or load the polyfill synchronously.