Allow multiple CCLA invitations to be sent at once

:fork_and_knife: You may now specify a comma separated list of email addresses to invite contributors to a CCLA.
@tristanoneil Just remembered that I created a method `manage_contributors?` that has the same logic in the CclaSignatureAuthorizer: https://github.com/opscode/supermarket/blob/master/app/authorizers/ccla_signature_authorizer.rb#L24

Could that just be used instead?