Feature/amd support

Here's my take on adding AMD support for skrollr. This approach supports module loading through require.js while keeping skrollr backwards compatible in case one wants to include it in a standard way through (script).

Demos:  [Amd version](https://dl.dropbox.com/u/9224326/www/skrollr/index.amd.html) and [Non Amd version](https://dl.dropbox.com/u/9224326/www/skrollr/index.html)