Add jquery-file-upload
Make client-side use jquery-file-upload
Process uploaded chunks.

   Also adds lockfile for asynchronous status updates.
Fix confusing message
Small review fixes.

  Comments, httpresponse -> jsonresponse; make sure errors get relayed.
Check download integrity
Add import tests
Split import-export into new file
Pep8 and pylint fixes
Behave properly on non-tar-gz files
remove repeated warnings
Handle nginx 499s and double requests
Pep8 and pylint fixes
Clear contentstore

Signed-off-by: Julian Arni <julian@edx.org>
Review fixes