Use proper default values for user config

The config/rapuma.xml file was being read but never used, and this was
causing some problems for brand-new configurations (e.g., new
installations of Rapuma). Fixed now.

Also improved the getConfigValue() function so it no longer needs to
call eval(), which could be a security hole. (Someone could edit the
rapuma.xml file to contain, say, the text "None; subprocess.call('rm
-rf $HOME', shell=True)" and the hapless Rapuma user would wipe out his
whole home directory when next he ran Rapuma. Not good.) Now it no
longer relies on eval(), and is thus much safer. (And more Pythonic.)