Get active student count from new Analytics DATA API

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@mulby @clintonb minor changes to be more consistent with the naming. Also updated the tooltip display per @lamagnifica lastest draft.

The requirements depends on making a first release to the data client repository.
The complete date range is exposed via the API, however the meaning of the dates in the interval needs to be communicated. It is a left-closed right-open interval. So 2014-06-13 to 2014-06-20 means that 2014-06-13T00:00:00 *is* included as is 2014-06-20T11:59:59, however 2014-06-20T00:00:00 is not.