Add surface API for streaming list-buckets.
Add implementation of streaming list-buckets.

* Feature detection for streaming list-buckets was added.
* HTTP bucket_list_path was modified to allow overriding the 'buckets'
  qs param.
* Since streaming list-buckets and list-keys use roughly the same JSON
  format, the common bits were factored out.
Modify surface API for timeouts and update a bunch of docstrings.
Validate timeout parameters.
Add timeouts to the backend and test streaming buckets.
Merge pull request #266 from basho/gh253-254-stream-buckets-and-timeouts

Conflicts:
	riak/client/operations.py
	riak/tests/test_all.py
	riak/transports/feature_detect.py