File permission bug while building .deb

[ConradHughes]

Great tool — thanks very much for creating it!

Encountered one problem while building: just using 'cp' to install files means that if the person who cloned or built cropgui has a private umask (e.g. umask 077), the files get put into the .deb with permissions that make them unusable after install. Switching from cp to using "install -m 0755" for executables and 0644 for non-executables should solve the problem,