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. 
Note that some of them must be blocking, at least if you want to ensure that dependencies are loaded in the right order (e.g. that `System` is defined when it's invoked, etc). I wonder if there's a way around it (much like the annoying `data-main` attribute used with requirejs).

Typo here too, btw.