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