fileMode 2, text, noise and noiseColor

Thank you for a perfectly simple and wonderful captcha generator.

1. I create a binary fileMode so source can pull captcha directly with <img src="/someRestInterface" />  I left the option named 'fileMode' to not break backwards compatibility.

2. I created an optional config.text option for times when the captcha value is known ahead of time.

3. Added noise and noiseColor options.