Issue #334, plan B:  'vendor-in' apitools subset.

- '__init__.py' files are stubs, w/o namespace support.  Deleted package-level
  'from <foo> import *' convenience imports.

- Non-'__init__.py' files pulled from:
  https://github.com/craigcitro/apitools/commit/e5a5c36e24926310712d20b93b4cdd02424a81f5

- Absolute imports converted to explicit relative.
Don't run pep8/pylint on vendored-in sources.
Avoid clash with same-named file.

Incorporates feedback from @dhermes:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/388/files#r20912015
https://github.com/GoogleCloudPlatform/gcloud-python/pull/388/files#r20912037