Add AuthorizeNet::Reporting::Transaction#get_unsettled_transaction_list

Added to Authorize.Net Reporting XML API in March 2011, see http://www.authorize.net/support/ReportingGuide_XML.pdf.

Hoping that you may be interested in updates, given that Authorize.Net appears to have orphaned the gem.  Was it you who published the gem to RubyGems.org?

- Piers C