Data for continuous quantities need time spans

[jshslsky]

Originally from @mschulkind.

Right now, data points only have a single timestamp, but this isn't really interpretable for continuous quantities like steps without knowing how long the data point is over.

For instance, if you see 100 steps, with a timestamp of 2013-11-15T00:00:00, is that 100 steps in one minute or one day?