Fixed notifications and icons

There is a bug in Bumblebee-Indicator.py

The notification doesn't disappear when it is showed. Fixed changing urgency level to URGENCY_LOW.

The icon on system tray doesn't show because you use a wrong relative path. Fixed with:

self.ind.set_icon(Config.icon_file_directory + "bumblebee-indicator.svg",'Bumblebee is unactive')
self.ind.set_attention_icon(Config.icon_file_directory + "bumblebee-indicator-active.svg",'Bumblebee is active')

This applications must be included in the most importante Linux distros. I would like to see how this project gets improve ;)