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

[gkurts]

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.

[kylewest]

this looks great. thanks for taking the time to do the work and submit the pull request.

looks like most of Program.cs is commented out ... I assume for testing. please revert those changes.

can you also update the readme with instructions for using the new methods you've added? don't forget to add yourself to the contributor list.

thanks again,

kyle

[gkurts]

Thanks. This is a very handy library. I'm using it in a side project that I'm working on and it is working great!

Yeah, I forgot about Program.cs ... was using it for testing before I decided to go ahead and write the actual test cases. I reverted it back and added a commented line in it for international shipments.

Added a unit test for multiple packages because I found & fixed a small bug in the international USPS provider (revision tag belongs outside of the packages loop.).

I also updated the readme a bit to talk about the new functionality.