Limit Gratipay's copyright license in Terms of Service (TOS)

[mattbk]

Reticketed from #204 (comment)

• Personally, I find using CC0 for Gratipay.com in §6.1 as a really nice touch and a fresh approach. I take it you do understand the implications of this, though.
• Staying on the topic of copyright, in §7.1 the copyright license that Participant grants to Gratipay is relatively broad to what Gratipay actiually needs at the time. While §7.2 clarifies what this license permits Gratipay to do, I would welcome a provision where it explicitly stated for which purposes Gratipay may use it.