Swish

The major change here is an implementation of Cancel Payment Request.

**Full Changelog**: https://github.com/MadSpindel/swish-python/compare/1.4...1.5

Add method to get QR Codes for app activation.

**Full Changelog**: https://github.com/MadSpindel/swish-python/compare/1.3...1.4

Small code cleanups in advance of some additions.

Updated Swish environments to reflect those available today: MSS (Merchant Swish Simulator), Sandbox, Production. Test, the old name for MSS, will be removed in version 2.0 (whenever that is made).
Added `callbackIdentifier` to fields to expect back in JSON responses.