Clean up find_transcript tests and add gateway_transaction_id to GatewayTransaction

This cleans up the tests related to find_transcript (formally find_transaction_transcript). It also adds the gateway_transaction_id to the GatewayTransaction class. 

Unfortunately, I am getting two remote test errors that I could not find fix for. I have added the test failures below.

  1) Failure:
test_invalid_login(RemoteFindTranscriptTest) [/Users/user/projects/spreedly-gem/test/remote/remote_find_transcript_test.rb:10]:
Exception message is incorrect.
<"Unable to authenticate using the given environment_key and access_token.  Please check your credentials."> expected but was
<"">.

  2) Failure:
test_successfully_list_payment_methods(RemoteListPaymentMethodsTest) [/Users/user/projects/spreedly-gem/test/remote/remote_list_payment_methods_test.rb:29]:
<"Y6CQL3mw3TPE9OcTa2x4EoCx6bm"> expected but was
<"3bAR8gtoPJ3ll3jddPgwH9y8RiF">.

73 tests, 214 assertions, 2 failures, 0 errors, 0 skips
rake aborted!
Command failed with status (2): [ruby -I"lib:test" -I"/Users/user/.rvm/gems/ruby-1.9.3-p327@spreedly_gem/gems/rake-10.1.0/lib" "/Users/user/.rvm/gems/ruby-1.9.3-p327@spreedly_gem/gems/rake-10.1.0/lib/rake/rake_test_loader.rb" "test/remote/**/*_test.rb" ]