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.
Add 'order' method to query.

Manually apply diff from Tagtoo-master:

    5f796e66a189fe1cef466cdffe23896358b78dc9^..
    4a772c709fda1420a36e42bd0c90a0bb2fc791c5
Add coverage for Query.order().

Fix bug uncovered by those tests.
Neaten docstring.
Add cursor handling to datastore.query.Query.

Manually apply diff from Tagtoo-master:

    25672e10fc359c67e0d267db2f23928b0a5761ca^..
    90ac07db1bcd3ebfd89f43ee9b59ded1cd8dd643

Add unit test, fix bugs.
Fix typo.

Incoporate feedback from @dhermes.
Rationalize docstring.

Incorporate feedback from @dhermes.
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
Resolving all sphinx warnings and parse errors.
Fixing remaining PEP8 long line issues.
Merge pull request #203 from tseaver/feature-datastore_query_order

Feature: datastore query order
Merge pull request #206 from dhermes/tox-docstring-fix

Sphinx and PEP8 docstring fix
Resolve conflict with master after 'order' feature landed.
Merge pull request #204 from tseaver/feature-datastore_query_cursor

Add cursor handling to datastore.query.Query.