Cut version 0.23.1 of Bigtable.

[dhermes]

Only commits since the last release came from #3089, so release notes will be easy.

$ git log bigtable-0.23.0..cut-bigtable-0.23.1 -- bigtable/
commit f99fb80baf810ca4447c7f1bc6b39a9f430f7072
Author: Danny Hermes <daniel.j.hermes@gmail.com>
Date: Fri Mar 3 11:18:05 2017 -0800
Cut version 0.23.1 of Bigtable.
commit 20ffd2be9b31bbff9bca8bd698c941da5a7f1951
Author: Danny Hermes <daniel.j.hermes@gmail.com>
Date: Thu Mar 2 16:54:21 2017 -0800
Sending x-goog-api-client header in Bigtable.
We missed this because it does not use GAPIC (the most
recent generated Bigtable GAPIC surface is 5+ months old).

[dhermes]

Suggested release notes:

google-cloud-bigtable 0.23.1

• Sending x-goog-api-client gRPC header with all requests for usage metrics inside Google (Sending x-goog-api-client header in Bigtable. #3089)

PyPI: https://pypi.python.org/pypi/google-cloud-bigtable/0.23.1

[dhermes]

Merging without CI, since there are no code changes.

[dhermes]

Eh, nevermind, no hurry.

[dhermes]

CircleCI is done, Travis has started. Yay for throttling builds!

[dhermes]

Here is the "push to PyPI" build: https://circleci.com/gh/GoogleCloudPlatform/google-cloud-python/1094