Support for gnome-shell 3.8

* getSummaryItems isn't defined anymore. Use getSources instead.
* set_style doesn't work anymore. Use 'style = style' instead.
* Sources have a new getter, indicatorCount, that returns the total
  notifications in there that are not transient neither resident.
  That's exactly what we need here.
* Gnome-shell 3.6 isn't supported anymore. A branch was created for it
  to be supported like 3.4.