Drop 'get_acl()' in favor of just using 'acl' property.
Make 'ACL' operations trigger a reload, if needed.

Fixes #338.
Merge pull request #373 from tseaver/338-make_acl_lazy_load-remove_get_acl

Fix #338: make 'acl' lazy load, remove 'get-acl()'
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