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.
Float '[source]' links right.
Make admonition blocks visible.
Tighten up 'note' sections (drop extra graph).
Remove 'docstring poetry'.

Wrap prose docstring paragraphs at 72 characters.
Remove spurious 'bulk_mutation' argument in docstring example.

Fixes #408.
Merge pull request #413 from tseaver/remove-docstring-poetry

Remove docstring poetry
fix api endpoints for storage upload/download
Merge pull request #414 from silvolu/fix-storage-requests-urls

fix api endpoints for storage upload/download
Pin oauth2client to 1.3

Update to 1.4 cause the following failure:
https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/43892995#L307
Unpin oauth2client after 1.4.4 release.
Merge pull request #415 from tseaver/unpin-oauthclient2-after-1.4.4-release

Unpin oauth2client after 1.4.4 release.
Add copyright and license headers
Merge pull request #416 from silvolu/copyright-headers

Add copyright and license headers
Changing auth to use implicit environment.