MerchantWarrior: Pass description

Merchant Warrior docs say the transactionProduct attribute is the
description:
http://cl.ly/image/0D0E3u3k1417/content.png

http://dox.merchantwarrior.com/files/MWE%20API.pdf

Use options[:description] like many other gateways to abstract this away
so we're not passing in custom options for each gateway.

Closes #945.