Replace hand-rolled upload/download impl. w/ apitools utilities.

Note that in order to use the 'Upload' / 'Download' utilities provided
by apitools without its 'base_api' module (not vendored), we have to unwind
some of the generated request setup code.

Fixes #334.
there should be a set of "do nothing" callbacks in apitools you can use here; if not, file a bug and i'll add them. ;)