Merge pull request #23 from mnastalski/multiple-channels

Add method to ease adding of transaction channels together

Merge pull request #22 from mnastalski/transaction-refund

Add transaction refund

Merge pull request #21 from mnastalski/channel-blik

Add new transaction channels

Merge pull request #17 from mnastalski/property-promotion

Use property promotion

Add sandbox link

Set encoding by default when registering a transaction

Add default timeouts to client

Fix invalid return types in TransactionStatusNotification

Use a string for API version

Add support for Guzzle 7.x