Add optional support for libnotify notifications.

Added suport for [libnotify](https://developer.gnome.org/libnotify/) as a compile-time dependency with the WITH_LIBNOTIFY flag, enabled by default. When not enabled, any use of the function will result in a girara style notification to the user explaining him that girara was compiled without it.

The notification will try to use the icon in the following order:
 - If there is a "named icon" such as the ones in /usr/share/icons, it will try
   to pick it up.
 - If there is an icon file, it will try to pick it up.
 - If there is nothing, it will use the "info" stock icon.

Refer to the "window-icon" and it associated callback for more information on icon handling.
If I launch 50 processes at the same time:
![fifty_girara_windows](https://cloud.githubusercontent.com/assets/1071217/3363947/be523740-fb19-11e3-8288-6cef0caafd28.png)
Seems to work fine.