Twikey-api-python

This release also includes the refund api + a minor fix while importing reporting files which doesn't return a json

**Full Changelog**: https://github.com/twikey/twikey-api-python/compare/v0.2.0...v0.2.1

Contains fixes for feeds (eg. resume on error)
Proper preparation of batches

* Improved private key usage
* Added invoice headers
* Improved naming

* Added private key to constructor
* Added extra headers to change behaviour of invoices
* Better naming

Corrections on various callbacks

Main highlights:

* Ability to update and cancel agreements
* Added url to a specific invoice