Fail CI if coverage < 100%.
Merge pull request #387 from tseaver/fail-tox-if-coverage-lt-100-pct

Fail CI if coverage < 100%.
Ideas commit for better (implicit) auth story in datastore.

Also
- Integrates this idea into the datastore regression tests.
- Creates a (temporarily ignored) cyclic import
- Has an uncovered statement that confuses nosetests
- Implements a friendlier (more gcloud-node like) version of
  allocated_ids
- DOES NOT update CONTRIBUTING to explain change to regression
  tests or ditch unneeded helper / environ code for regressions
- Creates a datastore cleanup script for when tests get
  broken