#295 let entity save accept null value for attribute
fix an issue that protoc buff will raise exception if a _pb object create but never assign value
Introducing custom pip install in tox to be used with cached wheels.

In addition:
- Removes used of "pip install" in Travis config with our custom
  pip install.
- Introducing tox environment "coveralls" to encourage all Python
  dependencies to be contained in "tox.ini".
- Puts LICENSE header back on update_docs.sh and cleaned up some
  bash long line / syntax with shellchecker.

See https://github.com/GoogleCloudPlatform/gcloud-python-wheels/pull/1#issuecomment-60648806
step 2 for context.
Removing overlap with tox.cover config in tox coveralls.
fix the helper function and test case while checking null value
Fixing comment description in update_docs.sh.
Merge pull request #296 from lucemia/support-null-value

#295 let entity save accept null value for attribute
Merge pull request #301 from dhermes/speed-up-build-stage-one

Introducing custom pip install in tox to be used with cached wheels.
Don't run 'regression' testenv by default.
Merge remote-tracking branch 'upstream/master'
Refactoring duplicate code between storage.key and storage.bucket.