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.
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'
Remove cyclic import from ignored for tests.
Reducing repeated pylint configs by defining test configs as a delta.

Uses ConfigParser to append modifications onto base file and
writes a new config file (which is gitignore-ed) before running
pylint.
Putting pylint additions for tests into run_pylint.
Implementing remaining datastore regression tests from gcloud-node.
Merge pull request #302 from dhermes/remove-cyclic-twice

Remove cyclic import from ignored for tests.
Uses pre-built wheels when pip installing from tox.

In addition:
- Creates script scripts/update_wheels_project.sh to kick off a new
  build when merging to master (outside of a PR).
- Moves update_docs into a larger group of merge scripts
  that happen when merging to master.
Merge pull request #307 from dhermes/finish-datastore-regression

Implementing remaining datastore regression tests from gcloud-node.
Merge pull request #308 from dhermes/speed-up-build-stage-two

Uses pre-built wheels when pip installing from tox.
Adding LATEST_COMMIT file with correct relative path.

Fixes error introduced in #308.
Merge branch 'master' into dev