Populate domestic and international services dynamically using USPS API

[sumo300]

Instead of hard-coding the domestic and international services in code, use the USPS API to retrieve the list of services for the admin. I had written an independent library a good while ago with the intention of maintaining a nopCommerce module for USPS using it, but never got around to really keeping up. Feel free to steal code if it makes it easier, but I know when working on it, the possibility existed to dynamically pull the list of services rather than having to release a new version any time USPS decides to add/remove/change services like in #9.

https://github.com/sumo300/seesharpship
https://github.com/sumo300/nop.plugin.shipping.seesharpshipusps