Fix issue #107:

Avoid calling setText() on every cell when MonthAdapter.getView() is called,
and only call setText() when the cell's content has changed.
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