Added support of U.S. possessions like Puerto Rico, Guam, etc. (USPS)

Added ability to calculate USPS rates for following territories: 

    AMERICAN SAMOA                  AS
    FEDERATED STATES OF MICRONESIA  FM
    GUAM                            GU
    MARSHALL ISLANDS                MH
    NORTHERN MARIANA ISLANDS        MP
    PALAU                           PW
    PUERTO RICO                     PR
    VIRGIN ISLANDS                  VI