Fix false assumption about scheduled basals from diasend

The original blip processing code was written under the assumption that it would have a continuous stream of scheduled basal segments.  That is not true for diasend and the logic that was associating the edges of scheduled basals was doing things wrong.  This fixes it to rely on the durations provided in the events, 'cause those are correct.
Merge branch 'master' into cheddar/fix-basal-associations
Merge pull request #49 from tidepool-org/cheddar/fix-basal-associations

Fix false assumption about scheduled basals from diasend