CAPTCHA module added

I have added the CAPTCHA module today due to the large amounts of spammers that have been signing up for accounts. Most of them seem to have Gmail accounts -- so, it was either CAPTCHA or block Gmail accounts from signing up. I'll start with CAPTCHA and see how that goes as I don't want to block legitimate users from signing up.

So far I have only enabled CAPTCHA on the registration page as spam has not been a problem in the comments section. If it becomes a problem, I will enable CAPTCHA side-wide.