Fixed incorrect use of rstrip()

rstrip() ate the last "r" in .rar files. 

"The chars argument is not a suffix; rather, all combinations of its values are stripped"
(see http://docs.python.org/2/library/stdtypes.html#str.rstrip)

Using rsplit() instead.
revert correct rsrtrip()
Merge pull request #75 from gonzalosr/patch-4

Fixed incorrect use of rstrip()