captchaCOLA: Show images for CAPTCHA validation with pictograms

This class can be used to show images for CAPTCHA validation using pictogram images.

It generates a CAPTCHA validation text from a random set of characters that can be made of letters, digits or both. The validation text is stored in a session variable for posterior validation.

The class renders a CAPTCHA validation image from pictogram graphics that are associated to each of the validation text characters.

The pictogram graphics are retrieved from image files stored in directories in the PNG format. Alternatively, it may use regular font letters instead of pictogram images.