Remove install-time requirements from 'setup_requires'.

That field is used for stuff needed at packaging time, but causes eggs
to be downloaded and installed in the project directory, which is
undesirable.
Merge pull request #230 from tseaver/no-setup_requires

Remove install-time requirements from 'setup_requires'.
Add explicit support for 'entity_value' in protobufs.
Lint removal.
Add helpers.set_protobuf_value.

Handles compount fields (entity, key) not assignable via setattr.
Use helpers.set_protobuf_value to support composite types.
Fix docstring param name.

Incorporates feedback from @dhermes.
Drop XXX comment.

Entities are not required to have datasets w/ active connections.
Don't set default key on entity protobuf if entity has no key.

Incorporates feedback from @dhermes.
Rename helper functions with leading underscore.

Signal that they are *not* part of the gcloud-python API.
Update doctring to indicate that helper functions are not API.

Note that these are functions, not methods.
Rename datastore.helpers -> _helpers.

Further emphasize non-API nature.
Merge pull request #232 from tseaver/support-entity-protobufs

Support entity protobufs
exclude demo from test coverage report
Merge pull request #234 from silvolu/exclude-demo-coverage

exclude demo from test coverage report
Removing gcloud.credential.Credential class.

No instances of this class were ever created and it only had
one factory method, which is now a function. This class was
also misunderstood / misreferenced in the docstring mentions.