Load mount/unmount icons from GTK+ theme

The icons are supplied as svg images and should be copied into
/usr/share/icons/hicolor
Add rasterization script using ImageMagick

The script makes png versions of the svg vector icons and places them in a
directory structure appropriate for GTK+ icon themes.
Add exception handling for absent icons

If the icons are not present in the GTK theme, they fall back to a default
supplied to the loading function.
Describe icon installation in readme
Improve loading of menu icon

The loading process has been encapsulated further. It also uses the theme icon
size instead of hardcoding it. To achieve this a more direct image constructor
had to be used.
Add icons for device locking actions
Explain manual GTK icon cache update
Add icon for detach action
Add menu icon for eject action
Reference Tango heritage of the icons
Install icons to system location in setup.py
Remove obsolete exception checks and default icon fallback code

The `gtk.image_new_from_icon_name` function does not raise an exception
in case it does not find the requested icon. Instead, it quietly serves
a default icon indicating to the user that the icon is not available.
This means that the execution branch that returns the default fallback
icon is not available as desired.

It should be noted that, since installing the icons in `setup.py`, it
should be reasonably safe to assume the icons are available.
Externalize responsibility to load icons from create_menu

Fix the following issues:

- icons were always loaded even if another icon set was provided as
  parameter to create_menu
- setdefault was used to modify the arguments which might have had side
  effects on calling code
Include icons in source distribution
Add missing `cp -r` parameter in README
Merge pull request #1 from coldfix/custom_icons

Custom icons
Merge branch 'feature/rasterize' into feature/custom_icons
Make menu icon borders more pronounced
Install and version rasterized icons for 16x16

Maintaining the rasterized icons as separate allows for fine-tuning after the
rasterization stage.
Change border colors of mount/unmount icons