Renamed "Challenge-Response-System" to CAPTCHA

[Admin]

The keyword "Challenge-Response-System" has been renamed to "CAPTCHA" on the front page of TrashMail.net.
I did this because some users were confused what means "Challenge-Response-System". "CAPTCHA" has become a very common keyword in the past time in the internet.
I hope it would help that TrashMail becomes easier to be used.