Vendor in apitools subset

Pulls in the subset of `apitools` which #334 claims can be used to implement upload / download in `gcloud.storage.key`).  These four modules do not use `google-protorpc` at all.

Updates the vendored-in modules to use relative imports (since they will only be used when the "real"
`apitools` is not available).

Updates our CI configuration to suppress PEP8 / lint checking for the vendored code.