Add optional 'container' prop to Package. Fix USPS ServerErrors never pa...

...rsing. USPSProvider: Add GetRates() baseRatesOnly param to turn of Revision=2. Detect LARGE packages based on dimensions or Package.IsOversized. Force valid container values when Package is LARGE. Set "Machinable" based on package dims.

This passes with the previously passing unit tests with the exception of the two USPS_Intl_Returns_No_Rates... The failure is caused by Assert.Empty(response.ServerErrors). The providers were not previously capable of parsing the Error element, so I'm not sure this is a good assertion anymore. Anyways, I want to get ServerErrors.