Add some sort of rate limiting

[daknob]

While this service is intended for use in Tor, rate limiting can be important in several occasions. Despite the fact that it increases complexity, some TorPaste hosts may see it as a requirement. Rate limiting in the normal web can be done per IP, by using Cookies, or by somehow tracking the user. In Tor, an environment for which TorPaste has been designed, such thing is either impossible or should not be done. For this reason, probably the best way to solve this issue is by some sort of challenge (Captcha). Let's see how this can be done and have an option to enable it in the configuration file.