Captcha Numbers V2.1: Generate CAPTCHA validation images

This class can generate images for use in CAPTCHA based validation tests (Completely Automated Public Turing test to tell Computers from Humans Apart). It is an improved version of the Captcha Numbers class originally written by Hadar Porat.

This version can generate a random text string that includes letters and digits.

It can also display each of text characters in different vertical positions randomly defined.

The colors of the grid and the text characters alternate to make it more difficult for robots to guess what is the text written in the image.

New features as of 2006 March 7th:
- random pixel display (by default set to 10% of the pixels of the entire
image); if random selected pixel has background color then a random color is
selected for the random pixel, otherwise a close match of the letter's or
grid's color is generated
- removed grid display by default (it's better readable without the grid,
but it can be added, my suggestion is to add more dots instead of the grid)
- random grid color support
- added random display angle support
- added random font support
- added random font size support
- added random font color support

Improvements:
- replaced rand function calls with mt_rand (mt_rand suppose to be better
and faster, if I remember correctly)

Next to the class PHP file create a folder fonts and place TTF in order to use the random fonts feature.