Fix USPS issues

The usps shipping calculators in this gem seem to be out of date. This PR does the following:

1. Add different service_code prefix for domestic and international rates because the code returned from usps API is not mutally exclusive between domestic and international rates. e.g. service_code "1" is priority mail for domestic and express mail for international. With the prefix we can now distinguish them.

2. Add two new shipping rates: GlobalExpressGuaranteed and StandardPost

3. Bug fix (check AVAILABLE_COUNTRIES instead of undefined WEIGHT_LIMITS in a few places)

4. Bring it up to Rails 4.0 and ruby 2.1.1

5. Fix broken rspec

6. Code cleanup