Added AMD module compatibility

I'm using this in a project where I'm managing dependencies with require.js. Added a fragment at top and bottom of screenfull.js that uses the amd `define` method to register the script if available, otherwise adds to the global namespace. I also had to tweak a few things here and there to protect global `window` and `documents` as well as maintain conformance with the jshint rules for the project.

For AMD compatibility I used a modified version of the amdWeb UMD module:

https://github.com/umdjs/umd/blob/master/amdWeb.js