Collect object / bucket updates in batch

[dhermes]

See
https://github.com/GoogleCloudPlatform/gcloud-python/pull/349/files#r19983607

By sending a PATCH request every time a property is set, we may make an application unnecessarily slow. We should work to make this simple and intuitive to users.

[tseaver]

I think "simple/intuitive" and "performant" are probably contradictory goals: distributed systems are hard.

[dhermes]

Sorry, I was just trying to throw this together quickly mid-review. Main thrust is just that we should consider batching.

[tseaver]

I didn't mean to sling stones: just noting that it is tricky to find a balance.

[tseaver]

#352 adds a quasi-transactional batch attribute, which can be used as a context manager to group multiple updates.