Fix issue #107

Avoid calling setText() on every cell when MonthAdapter.getView() is called.
This appeared to be the cause of all the messages like:
09-25 16:31:18.499  W/View﹕ requestLayout() improperly called by com.squareup.timessquare.CalendarCellView{b32631b8 V.ED..C. ......ID 408,0-476,68} during layout: running second layout pass