refreshing the list of tasks on day change

It is a simple fix to address issue #100 at least in most cases (when user returns to the application after midnight).
It is done by:
* checking in `onResume()` if the date has changed and if yes sending a refresh broadcast
* clearing `dateCache` in `notifyDataSetChanged()`, because the date could have changed.

In general I'm not sure if `dateCache` is really necessary, because `TaskAdapter#formatDate()` is a pretty fast operation.