Allow multiple CCLA invitations to be sent at once

You may now specify a comma separated list of email addresses to
invite contributors to a CCLA.
Change InvitationAuthorizer to OrganizationAuthorizer

Since the InvitationAuthorizer is actually used to ensure that the user
has the correct permissions for a particular Organization this renames
the InvitationAuthorizer to OrganizationAuthorizer and updates the
OrganizationInvitationsController where it is used.
Combine OrganizationAuthorizer and CclaSignatureAuthorizer

Since the organization is the gatekeeper of CCLA signatures it seems
logical to move the the authorization logic to OrganizationAuthorizer
this also allows the logic to be shared for organizationy type actions
like managing contributors. This also adds a new authorized action,
resign_ccla? which protects a CCLA from being resigned by a regular
contributor in an organization.
Merge pull request #489 from opscode/multiple-ccla-invites

Allow multiple CCLA invitations to be sent at once