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. 
@theefer Yes you are indeed correct, people place at the bottom to avoid blocking rendering as html its parsed incrementally, but the dom ready event is not fired until the entire dom is parsed. Declaring the script as async removes this dependency.

@OliverJAsh regardless of the code inside being async (Sysytem), the browser will still treat its invocation as synchronous. Modern browsers have speculative parsers (i.e. the "look-ahead" or "pre-parser") which actually favour async elements, and therefore applying this may actually speed up execution.