Captcha

Hey! I didn't find gem that supports mongo-mapper, so I had to extend simple_captcha :) Unfortunately, the author of the original gem is inactive more than one month, so we are using https://github.com/ArturG/simple-captcha version in the Gemfile. Probably it is not the best solution, so feel free to fork my repo and replace the link (to get rid of dependence). Thanks

Preview:
![screen shot 2014-02-21 at 11 38 11 pm](https://f.cloud.github.com/assets/1472617/2236994/bcc883d4-9b7e-11e3-81ac-a5bf35326cf7.png)