Updated datastore.entity with pyfmt.
Updated datastore.entity by hand until fully passed pep8.
Resolved all pylint errors in datastore.entity.

Pylint errors were:
  C: 90, 0: Line too long (82/80) (line-too-long)
  W: 67, 4: __init__ method from base class 'dict' is not called (super-init-not-called)
  C:138, 4: Invalid argument name "pb" (invalid-name)
  E:177,43: Instance of 'Entity' has no 'to_protobuf' member (but some types could not be inferred) (maybe-no-member)
  E:191,19: Instance of 'Entity' has no 'to_protobuf' member (but some types could not be inferred) (maybe-no-member)
  E:196,27: Instance of 'Entity' has no 'is_partial' member (but some types could not be inferred) (maybe-no-member)
  E:202,18: Instance of 'Entity' has no 'path' member (but some types could not be inferred) (maybe-no-member)
  E:216,51: Instance of 'Entity' has no 'to_protobuf' member (but some types could not be inferred) (maybe-no-member)
  E:221,38: Instance of 'Entity' has no 'path' member (but some types could not be inferred) (maybe-no-member)
  R: 24, 0: Too many public methods (25/20) (too-many-public-methods)
  W: 18, 0: Unused import datetime (unused-import)
Fix #136:  roll back transactions on exceptions.

Cherry-pick:  3221122ad1eff0e96636b440d996226c737890e8
Fix typo, s/derault/default.

Feedback from @silvolu.
Clarify sentence.

Feedback from @silvolu.
Merge pull request #202 from tseaver/136-rollback_on_exception-redux

Fix #136: roll back transactions on exception (redux)
Merge pull request #189 from dhermes/pep8ify-datastore-entity

Make datastore.entity PEP8 and pylint compliant