Fix: make jobtype_root_package_data_roots a tuple

This fixes a bug where get_package_data may get into what seems like an
endless loop to the user.  When jobtype_root_package_data_roots is a
string, get_package_data() will eventually be called with the string
"pyfarm/" as the argument.  Instead of looping over several directories,
it will now loop over the string "pyfarm/".  Since the last character in
that is "/", it will eventually call os.walk() with "/", causing it to
try to include the entire VFS into the package_data, including stuff
like /dev, /sys and /proc.
Safety check in get_package_data()

This will make sure get_package data is not called with absolute paths
Make print statement more Python 3 safe
Merge pull request #109 from guidow/setup_py_bugfix

Setup py bugfix