Add 'eventual' flag to 'Connection.lookup()' / 'Connection.run_query()'.

Allows selecting EVENTUAL consistency, rather than the STRONG default.

Fixes #305.
I used `datastore_pb.QueryResultBatch.NO_MORE_RESULTS` in #432. Would
```python
more_enum = datastore_pb.QueryResultBatch.MoreResultsType
more_enum.Value('NO_MORE_RESULTS')
```
have been the right way to do this?