Add backtracking to the PathMatcher

Hello @dpup, @vinibaggio, 

Please review the following commits I made in branch 'nick-path'.

43538ec9836a1d8a63b598843a88c7505603e482 (2013-09-27 10:50:17 -0700)
Add backtracking to the PathMatcher

Check list:


R=@dpup
R=@vinibaggio
VISIBLE_CHANGES=Add backtracking to the PathMatcher