Special emergency patch for "stealing first" play.

With this patch, events in the game between the Cubs and Brewers
on 2013-04-19 can be accommodated.  In that game, Jean Segura
"stole" first base after having attained second base safely.

This commit now permits the "advance" of a runner backwards from
third base or second base.
Update version number to 0.6.1.
Merge branch 'maint' of github.com:chadwickbureau/chadwick into maint
Correct latest version in index of documentation.
Merge branch 'maint'