Update to 5.x

Here's an initial patch that attemps to update this module to 5.x. Unfortunately, this is probably as far as I'll be able to take it because it doesn't look like this module will fit our use case, but hopefully it gets someone started in taking it the rest of the way. :)

Known issues:
- Most importantly, the module doesn't actually work. ;) Saving the taxonony_similar form results in some errors because $form_values['taxonomy'] is not the same as the old $edit['taxonomy']; as a result, any tags attached to a node are deleted.
- The checkboxes should probably be smart about their default values.
- "Similar" tags never came up for me. Neither did the not similar warning. Not sure if that's because I didn't have a wide enough range of test data or what.
- I'd love to replace the hook_nodeapi with a simple #redirect, but that appears to be impossible since you don't have a $node->nid until after the node's been inserted.
- I'd also love to take those divs out of the element attributes and place them into a form theme function instead for optimal code separation.