Fixes #2096. Non-free Duality.ttf removed. Open/free Captcha.ttf added.

Fixes #2096. Non-free Duality.ttf removed. Open/free Captcha.ttf added.

## Generated CAPTCHA samples

![captcha-samples](https://f.cloud.github.com/assets/100198/240864/2e1cc9c6-8958-11e2-89bf-f746265137b6.png)

## New font glyph set

![captcha-glyphs](https://f.cloud.github.com/assets/100198/240865/2eb9f57a-8958-11e2-8358-ad0bc2a4e5ba.png)