Add Commercial Base Rate Support

Many USPS shippers use the Commercial Base rate instead of the USPS Retail rate. Commercial Base rates are available for all online postage creation, including Endicia and USPS's own online "Click-N-Ship".

This adds an option to enable the use of those prices instead of retail rates.

To use, just add it when you create the carrier like so:

    ActiveMerchant::Shipping::USPS.new(login:ENV['USPS_KEY'],commercial_base:true)

Fixes #60

TODO: Commercial Plus and it's various contracts. (ie. one can be Plus for Priority Flat Rate Envelopes and regular Base for everything else.)