Make twoWeek.data() more flexible with viewEndDate

If no viewEndDate is passed, default to the most recent `days` element (vs
the last datetime of the data set, which could be outside range).

If a viewEndDate is passed, allow it to be a string (vs only a Date
object).
Merge pull request #7 from tidepool-org/nicolashery-patch1

Make twoWeek.data() more flexible with viewEndDate