NEMO-3258 Correct rates returned from USPS.
Enhanced cache key to include the max_weight allowed to ship to a country
Modified compute_package method to allow callback to identify desired rate from rate_result hash to eliminate need to override method in USPS base class.  This eliminates an extra call to rate service when method does not support destination.
Modified check methods to properly handle a nil value.
Merge pull request #3 from godaddy/3258

NEMO-3258 Correct rates returned from USPS.