Bug fixes and improvements

I faced a few problems using svn-stash in a Cygwin environment:  

The `svn-stash pop` command always throws **rm: cannot remove blablah: No such file or directory** errors *(2 times to be specific)*. The corresponding method should not delete the stash files as it's already followed by a `delete_stash()` call in `execute_stash_pop()`.  

Another issue is that this command always fails in patching stashed files, replacing `patch -p0 < FILE` by a `svn patch FILE` resolved my problem.  

The following pull request should solve these two issues, I couldn't find the time to test it on another platform but it works fine in Cygwin.