Negative item amount (#15)

* added: set $negativeAmountAllowed to true in PurchaseRequest

* removed .idea folder

* restore .gitignore file

Fix for issue: No merchantid #11. (take 2) (#13)

* Add header to fix issue #11.

* Add header to fix issue #11.

* PSR2.

* PSR2.

Allow 5.6 and 7.0

Update readme

Resolved issue #10

Sisow now doesn't return an invalid sha1 warning anymore.

Update README.md

Store error message in $message

Fixes #6

Merge pull request #1 from Enrise/feature/fix-completepurchase

Make the complete purchase request never redirect

Update readme