Added support for HTML5 File Access API

Added support for HTML5 File Access API by creating stubs for Blob, BlobPropertyBag, File, FileList, FileReader, DOMError, ProgressEvent. Also exposed the classes in the Window class.

I've implemented the functionality by following what's listed under: http://www.w3.org/TR/FileAPI/, MDN and by looking at what's exposed in javascript when I manually instantiated these objects.

I've followed the coding guidelines and tested this locally. Since this is my first major checkin, let me know if there's anything that needs to be changed or clarified. Thanks!