Fix AMD support

The registration of picturefill via AMD as
an anonymous module was broken. It was attached global to `window`. This branch fixes it.

* Fixes the AMD detection
* Fixes `w.picturefill();` is `undefined` if registered via AMD
![bildschirmfoto 2014-05-13 um 16 53 05](https://cloud.githubusercontent.com/assets/464300/2959617/cfd96d1a-dab4-11e3-9d44-dbaefe2df269.png)