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.
(Minor correction to Gabe's comment:  2014-06-20T11:59:59 is not included, but 2014-06-19T11:59:59 is.)