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.
Use 'Upload.ConfigureRequest' to decide 'simple' vs. 'resumable'.
Appease lint.
Garden in 'python-modernize' updates to apitools.

Catches up our vendored-in source to commit:
5a1bab1df7a474cff57f7f5cc066b19a1c286a21.
Add a regression test for small-file writes.

In conjunction with the existing large-file test, this one ensures
that we exercise both the 'simple'/'media' upload API, and the
'resumable' one.
Merge pull request #407 from tseaver/334-use_apitools_transfer_for_upload_download

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