Set config.global_id.verifier to nil if #message_verifier raises
Squashed commit of the following:

commit d38db7346954fc342edbea7b18ddc6cce3088fe4
Author: Dan Olson <olson_dan@yahoo.com>
Date:   Mon Aug 18 22:07:10 2014 -0500

    Remove unused variable in test and update assertion
Ensure SignedGlobalID.verifier is reset properly
Rescue ArgumentError specifically
Allow a verifier option to be passed to SignedGlobalID. Raise in SignedGlobalID.parse when verifier is nil as well
Merge pull request #19 from DanOlson/no-verifier-on-init

Set config.global_id.verifier to nil if #message_verifier raises
Signed Global IDs accept a :verifier and default to SignedGlobalID.verifier