Major overhaul of image captcha - Whitespaced

This is a big patch and I know this is not very appreciated by the maintainers ;).
But most changed things are related and it was too difficult to isolate stuff while I was busy coding

Anyway, here are the things that are in the patch:

• Added and worked on error checking an reporting. For example, the patch now even makes it impossible to select an image captcha when there is no font configured. Also solved a bug where the module would wrongly report that there was no font selected (typically during first configuration)
• made image module utf friendly: module also works with non ASCII characters like ë, ç, á etc (see http://drupal.org/node/164616). Also added the check that there shouldn't be spaces in the character list (see http://drupal.org/node/169494)
• Cleaned and streamlined the image generating code. There appeared to be a lot of cruft in the code and things could be done easier/more compact/more efficient.
• Added an option for a less resource hungry image generation by disabling bilinear interpolation
• Added options for the image captcha: font size, distortion level, noise level.
• Added the example image again on the settings page, which makes it much easier to configure the image captcha (also see http://drupal.org/node/165577)
• Solved problems reported by the coder module (also see http://drupal.org/node/170203)
• various minor tweaks

I'll post a screenshot in follow up.