Allow project maintainer to manually add contributor email addresses

[sumbach]

Until all the legal details get worked out (see #5), it would be great if CLAHub could also support other CLA signing processes (e.g., mailing a signed paper copy of the form, fax, S/MIME or GPG signature, etc).

The easiest way to allow projects to effectively use CLAHub (and GitHub pull requests) is the provide for manual entry of CLA signing info:

• date of signature
• name
• email address
• GitHub username?
• mailing address?
• phone number?

See also #25, #31.