Update today variable when refreshView() is called

I noticed that when I called CaldroidFragment.refreshView(), and it was a new day, it did not change the background color / drawable for the date cell for the new day.  So I added an `updateToday()` function in `CaldroidGridAdapter` that is called when `CaldroidFragment.refreshView()` is called.

I have a BroadcastReceiver in my project that is called at the start of each day, and I call refreshView() in it, so now the calendar automatically updates the background of the cell for the new day.