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.
https://github.com/GoogleCloudPlatform/gcloud-python/commit/2a306feb54e33ad221c8fd8c3be96efcfd94a103 implements what I described.