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

Allows selecting EVENTUAL consistency, rather than the STRONG default.

Fixes #305.
Spell enumerated value more naturally.

Feedback from @dhermes:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/449/files#r22115879
Improve variable names.

Feedback from @dhermes:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/449/files#r22116090
DRY out read_options logic.

Feedback from @dhermes:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/449#discussion_r22116623
https://github.com/GoogleCloudPlatform/gcloud-python/pull/449#discussion_r22116657
Merge pull request #449 from tseaver/305-lookup-run_query-eventual

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