plugin: pass through conf_dir from Kill Bill

Kill Bill will now give a new parameter to plugins, which is
a location of a directly containing configuration files.
This can be used to de-couple plugins code from configuration.

By default, this value will be set to the plugin directory (root)
for backward compatibility.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
Jarfile: update to Kill Bill 0.1.75

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>