Rename datastore._helpers -> helpers.

We are going to move 'Entity.from_protobuf' and 'Key.from_protobuf' into
the module as free functions, which will be APIs:  this module needs not
to signal that it has no APIs.

Document now-API module 'gcloud.datastore.helpers'.
Fix #260: remove import cycles in datastore.

'entity.Entity.from_protobuf' classmethod -> 'helpers.entity_from_protobuf'.

'key.Key.from_protobuf' classmethod -> 'helpers.key_from_protobuf'.
Enforce cycle checking.
Update 'gcloud.datastore.helpers' to indicate API-ness.

Correct docstring.

Add '__all__' to indicate public members.
Use longer name for list-comprehension placeholder.

Incorporate feedback from @dhermes.
Merge pull request #289 from tseaver/260-break_datastore_cycles_move_from_protobuf

Fix #260: break datastore cycles by moving 'from_protobuf' classmethods
Test adding encrypted key.
Test commit to determine if regression is run.
Re-encrypt to make sure file correct.
Putting back original regression test.
Squashed commit of PR #281 in main repo.
Removing most of Travis for test branch.
Removing uses of tox to try to speed up BUILD.
Small tweak to ditch tox and install library on Travis.

See previous commit for context:
5277854a79b8dbce76062ac7dc7945a76b6dd679
Make pep8 ignore the pb auto-generated file.

See previous two for context:
823976f37ea95165e92b7254d5ef2e897896831b
5277854a79b8dbce76062ac7dc7945a76b6dd679
Helping Travis import Google protobuf.

See previous for context:
1036c852ac1f21fbb3625c24739f36d6a3ede285
823976f37ea95165e92b7254d5ef2e897896831b
5277854a79b8dbce76062ac7dc7945a76b6dd679