Set permissions on installed files, especially Python modules.

Files are set to rw-r--r-- and directories and executable files to
rwx-r-xr-x.

I ran into a permissions problem when installing Rez a while back
where not everyone on the network was able to read the files (the
permissions in my home directory, where I was installing from,
was more restrictive).  I'm not saying that this pull request is the
perfect solution, but it is something to consider.