Fix for error occuring on selecting carrier. #6456

  * When a shipment is created from scratch, and a carrier is selected,
`on_change_carrier()` leads to `get_ups_shipping_cost()`, which should
ideally not be called as there are no values yet inside our unsaved
shipment.

  * In `get_sale_price()`, the fix is to check that the given shipment
ID is a positive integer. This indicates a saved shipment.
Added decorators to some methods. #6456
Merge branch 'release/3.2.1.8' into develop
Merge pull request #48 from PritishC/feature/task-6456

Made fix for error in `on_change_carrier()`. #6456
Replace python logging with logbook #6500

Added logging for address_validation
Merge pull request #52 from tarunbhardwaj/develop

Replace python logging with logbook #6500
Migrate to 3.4 #6575

Ref: GH-55
Merge branch 'pr/55' into develop

Ref: GH-55
Released version 3.4.1.0
Merge branch 'release/3.4.1.0'