general improvements

[sbussetti]

did some refactoring. main points:

1. ChunkedUploads now provided as a model field. Acts as a drop-in replacement for a generic FileField.
2. Most javascript packaged up as a widget media on the field's default widget now.
3. Updates vendor js libs for browser compatibility

Still some to-dos as far as making things more configurable; probably follow up w/ that later.