Replaced JsZip with Dart package:archive.

Now the submodule for JsZip has been removed, as has the need for any
JavaScript code (except the background page). Therefore the jsProxy and
the .o hackery has been cleaned up.

Also now the code requests tar.gz files from the server instead of
uncompressed zip files. I've tested that now tavern works perfectly
with the pub server. Addition permissions were added to the manifest
file so that downloading from pub is working.

R=tapted@chromium.org

Review URL: https://codereview.appspot.com/58050043