Added USPS International Provider, single service rate option, and some unit tests

I've added a USPS International Provider, changed the individual USPS and UPS providers to accept a single service type for a one off rate lookup rather than just doing rate shopping, and added some unit tests to the project (didn't touch FedEx.. yet). Would like some feedback if you think the changes I made are worth pulling into your main branch.

![image](https://f.cloud.github.com/assets/708797/2056011/6dcfa05a-8ad0-11e3-9ad7-a278f6468fba.png)