Customize all options, support for background images

I started using captcha/textimage recently but was disappointed in the options offered by Drupal's captcha as opposed to other captchas I'd used in the past. It looks like a lot options are available, but they don't have settings offered yet (except the font-size, recently added).

I cooked up a patch that adds simple options to customize the features of textimage, with form verification and a few new options. I tried to make the patch work in all situations, including when there are no background images or TTF fonts. Most settings affect captchas with or without TTF support equally.

Here's the list of changes:
* Text doesn't run off the image to make unreadable
* Added settings for all available options
* Added support for background images (works better with lighter images, since the text is usually dark)
* Added new options noise (using pixels) and image margin
* Most options work with generic fonts for systems that do not offer TTF support

I've attached a screenshot of the captcha with font size 24 and with and without image backgrounds. The patch will follow shortly.