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 I believe this was deployed to Staging, and I think it has caused an issue by the removal of `CclaSignatureAuthorizer#show?` which is used here https://github.com/opscode/supermarket/blob/master/app/views/ccla_signatures/contributors.html.erb#L7