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. 
@phamann I'd like to know more about all of this!

What do you mean "it will still treat its invocation as sync"? JS invocation is non-blocking (or so I thought). According to [MDN](https://developer.mozilla.org/en/docs/Web/HTML/Element/script), `async` "has no effect on inline scripts (i.e., scripts that don't have the src attribute)."

> 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.

Where can I learn more about this? How will scripts with the `async` tag be favoured by speculative parsers?

Evidently, I don't know enough about how JS is parsed by the browser. I'm going to merge your PR, but I just want to squeeze some info out of you :wink: