Expose the 'more_results' value fetched from the back-end.

Include notes that it is known to be currently un-useful, since the
back-end sets it to indicate there are more results willy-nilly.

Fixes #423.
Mention versioning policy in README
Merge pull request #426 from silvolu/version-policy-readme

Mention versioning policy in README
Use 'pathto' to load static resource
Expose 'missing'/'deferred' in 'Connection.lookup'/'Dataset.get_entities'.

If 'deferred' list is not passed, the connection retries any keys in
a deferred response.

Closes #306.
Merge pull request #425 from tseaver/423-expose_query_more_results

Fix #423:  Expose the 'more_results' value fetched from the back-end.
Merge pull request #428 from silvolu/fix-docs-static-resources

Use 'pathto' to load static resource
Changing Query.filter to ask for operator and prop. name separately.

Also adding a test for property names with spaces in them.

Fixes #424.
Remove note about sleep.

@pcostell says not to bother.
Move copy-deferred-keys-to-request logic out of 'Connection.lookup'.

Addresses @dhermes comment:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/429/files#r21997643
Squash checks of protobuf call headers into helper method.

Addresses @dhermes comments:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/429/files#r21998236
https://github.com/GoogleCloudPlatform/gcloud-python/pull/429/files#r21998714
Document that / lists passed as arguments must be empty.
Merge pull request #430 from dhermes/fix-424

Changing Query.filter to ask for operator and prop. name separately.
Enforce that / lists passed as arguments must be empty.
Dead chickens on the altar of pylint/pep8 continuation taboos.
Merge pull request #429 from tseaver/306-connection_lookup_missing_deferred

Expose 'missing'/'deferred' in 'Connection.lookup'/'Dataset.get_entities'
Stash.