Drupal 6 support + additional features

Here's a port of the module to Drupal 6. Because of the huge amount of changes I'm only providing the complete files, not a patch.

This version includes several changes and new features. Some of which are discussed at and taken from the patches of #214886: More Functions, a settings-page, a german-translation, ....

New features compared to the original:

• Checkbox to make licence selection mandatory or optional
• Dropdown to select a default selected licence
• Multiple select dropdown to make a selection of licences to choose from
• Dropdown to select the licence jurisdiction

Changes compared to the original:

• Drupal 6 support
• Extended descriptions
• Code clean-up and simplification
• UI clean-up
• Improved the .install file by using hook_schema and drupal_install_schema() to support all db's and removal of variables
• License -> Licence :-P
• Removed 'Developing Nations' licence
• All text is translatable
• Minimized output block text

Note that the db schema has been changed as well so it's not compatible with the previous versions of this module.

Open issues (see #214886: More Functions, a settings-page, a german-translation, ... for details):

• Should we be able to specify a licence version? If yes, how should we implement this?
• Should one or more of the new options be moved to the node edit page? If yes, which?

To-do:

• Option to select the licence image style used in the output block
• Custom output block text(?)
• ...or just an option to only display the licence image(?)

...I think that's about it...