Fix recaptcha.net instructions
Add captchas.net python module

This is from http://captchas.net/sample/python/CaptchasDotNet.py
Add missing import in captchas.net python module
pep8 captchas.net python module
Remove link from captchas.net image

This link is disturbing to users. The attribution banner is still
displayed.
Add captchas.net captcha verification

Due to privacy and usability concerns, captchas.net is added as another
captcha solution besides reCAPTCHA.

This makes use of the provided python module on captchas.net [0],
however only uses parts of its functionality. In particular, we use the
pylons session to store the random tokens instead of files. These files
are however still created and cleaned up without any need.

[0] http://captchas.net/sample/python/captchasdotnet