Add icon support

- icons can be displayed at the top left or right corner
- disabed by default, fully backwards-compatible
- no additional dependencies, only cairo used
- TODO: only supports png (but all default images are png)
- see issues #26 and #139

You can **try this pull request** by building dunst from source:
1\. Enable icons in your [dunstrc](https://github.com/Gjum/dunst/blob/feature-icon/dunstrc#L141):
``` diff
 [global]
+    # Align icons left/right/off.
+    icon_position = left
+
+    # Paths to default icons. Make sure these exist on your system.
+    icon_folders = /usr/share/icons/gnome/16x16/status/:/usr/share/icons/gnome/16x16/devices/
```
2\. Run the following commands to rebuild from source and reconfigure dunst:
```
git clone https://github.com/Gjum/dunst.git
cd dunst
git checkout feature-icon
make
sudo make install
killall dunst
```
3\. Try if it works: `notify-send "some text" -i /path/to/your/icon.png`

Make sure to comment below with your feedback!