deacaptcha: Display CAPTCHA images and validate forms

This class can be used to display CAPTCHA images and validate forms that use them to verify whether the user is really a human or a robot.

It can generate an image with a random text taken from a text file.

The text is rendered using TrueType fonts over a background of random faded text letters and background images.

The generated image is stored on a file in GIF, JPEG or PNG formats.

The validation word is stored in a session variable for posterior verification.

This dimensions, the fonts, and the background images are configurable.