----- 3.1.0 under development -----
    - add price quotation functionality
----- 3.0.0 -----
    - support for PHP 7+ only
    - improved shipping response
    - user may now re-use received TNT key to obtain shipping at any time.
      Key is valid no longer then 26 days.
    - tidy up code
    - fixed known bugs

----- 2.0.2 -----
    - restored PHP 5.6 compatibility