Use xrescat in PATH

[zackattackz]

Before this commit, rofication/_util.py:Resource would call the command /usr/bin/xrescat. After this commit, only xrespath is called, which should use the user's $PATH to determine the location of xrespath.

This change will allow for more flexible installations of xrescat. It will make packaging rofication in other linux distributions that doesn't use /usr/bin much easier.