MINOR: linux: automatically detect the installation path

Do not force users to install anathema on /usr/share/anathema

This patch will use standard shell scripting to detect the real
anathema installation folder.
MINOR: linux: allow to specify a custom repository path from command line

When invoking anathema.sh if an argument is provided this will
override the default repository path
Merge pull request #370 from nolith/anathema_script

Linux anathema.sh overhaul