Add captchas.net captcha verification

Due to privacy and usability concerns, *captchas.net* is added as another
captcha solution besides *reCAPTCHA*. Both are only used for new user
registration.

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.

Furthermore the audio captcha link could make use of an icon and HTML5 audio and such.

To make use of this feature you need to create an acount on [captchas.net](https://captchas.net/registration/) and add the details in the adhocracy
configuration.

[0] http://captchas.net/sample/python/captchasdotnet