Allow setting 'at' value on Datastreams.

This is permitted by the API, and makes certain types of updates simpler
to code, i.e. when you want to send your own timestamp for a single
value you know don't have to create an extra Datapoint instance to wrap
that update in.
Bumped version to 0.1.0-rc1
Fix indentation
Merge pull request #9 from smulube/datastream_timestamps

Allow setting datastream timestamps on create or update