Educate users about binary mode.

Follow-up to #7 

Unsure responses and CAPTCHAs are a rather important aspect of how Mollom works.  The current configuration option is not documented at all, causing users to blindly change it without understanding consequences.

![2013-07-30_20h16_53](https://f.cloud.github.com/assets/41992/880900/44427370-f944-11e2-8d11-fc4807da20fc.png)