Proxy server support

In my current project I have a need for proxy server support in active_shipping.  This code fixes my issue and will hopefully be helpful for others in the various projects which use active_utils.  To use it one simply needs to set the "proxy" class variable on ActiveMerchant::Connection:

ActiveMerchant::Connection.proxy = 'http://server.proxy.domain:1234'

Let me know if you have any questions or suggestions for improvement of this patch