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.
Is [`ConfigureRequest`](https://github.com/GoogleCloudPlatform/gcloud-python/blob/master/_gcloud_vendor/apitools/base/py/transfer.py#L515) part of the setup that we have to unwind? You're forcing the upload to be always resumable; `ConfigureRequest` provides some optimization for it.