Add ability to retrieve a shipment's tax total and final price
Remove pending from API payments test which seemingly works OK
[backend] Display payment state on payments/show
Added partial capturing and payment capture events

After authorization of a payment you can capture either the full amount or a partial amount. The full amount can be captured using purchase! or capture! with no arguments. A partial amount can be captured with capture!

By using the 'capture!' method, a payment capture event will be logged so that we can track what amounts of a payment have been captured and when.
Use decimal field for capture_events, just like payment table
Store uncaptured amount when capturing + purchasing, and log capture events when purchasing also
Add uncaptured amount to payments table
Revert "Remove pending from API payments test which seemingly works OK"

This reverts commit 12218bcd63614db7d2d7c69c3f0ae2a36cc33c5f.

This still raises the problem on the CI server. TBI later.