Move storage code into a subpackage

[dhermes]

This PR was created via: https://gist.github.com/dhermes/e239691aa584bd56a5352e34aad27cf3

export PROJECT_DIR="${HOME}/google-cloud-python"
export READMES_DIR="${HOME}/i-wrote-some-readmes-for-2357"
cd ${PROJECT_DIR}
git worktree add -b make-storage-subpackage ../hotfix official/master
python make_commits.py \
--git-root "${PROJECT_DIR}/../hotfix" \
--package storage \
--package-name "Google Cloud Storage" \
--readme "${READMES_DIR}/storage/README.rst"

[theacodes]

LGTM with a few tiny things.

[theacodes]

LGTM

[tseaver]

• Won't the chnages to top-level files conflict with Move bigtable code into a subpackage #2410?
• The duplication across the sub-repositories (.coveragerc, MANIFEST.in, etc.) is getting worse: to we have an issue to track stripping that out?
• Likewise the duplication inside the top-level stuff (.travis.yml, tox.ini).

[dhermes]

@tseaver Yes essentially all will conflict, but luckily I can just re-run the script (that is why I ran it).

• The duplication of .coveragerc can be wrapped into a single file (put it in [TRACKING] Move all packages into subpackages #2404 to track)
• The duplication of MANIFEST.in is essentially necessary, just like do-nothing __init__.py files are. Also note that over time these files may take on a life of their own
• The top-level stuff in (tox.ini and .travis.yml) will be greatly reduced when I address Only run tests for modules affected by a PR #2277

[tseaver]

OK, LGTM

[dhermes]

Pushed to resolve rebase conflict and re-ran script. Won't wait on Travis since a) that could be awhile and b) this will be painful.