Code improvements, interface polishes.

Further refactor and clean up the code. Add tests (mostly
DateUtilsTest). Complete the documentation for LanguageResource.

* Actually display number of days until marathon based on current date
  selection.

* Change date string for current activity to be DateFormat.MEDIUM (and
  locale based)

* Add missing French accents in existing messages.

* Change the training activity string to display the kilometers, THEN
  the activity type rather than activity type then kilometers (i.e.
  "Regular: 4 KM" becomes "4 KM Regular").

* Switch argument order for getNumberOfDaysBetween(date1, date2).