Added dream factor.

Dream factor is a measurement of a character's upset potential. It is
the number of fights a character has won against the odds over their
total number of wins. Dream factor percentage will now be displayed with
other stats such as win/loss percentages and streaks.

SaltyStats will automatically upgrade statfiles from earlier versions to
work with 0.4.0. The upgrade process will also add a version number to
the statfile to make future upgrades easier.