Fix signature encoding

Merge pull request #25 from nerds-and-company/fix_generate_signature

Fix generating signature by urldecoding value

2.1

Incorporate several PRs since the last release:

• Sepa direct debit
• Add getTransactionId to CompletePurchaseResponse
• Add support for reject and error URLs

v2.0.2: Merge pull request #17 from DennisHendrickx/master

Includes PRs since the previous release.

v2.0.1

Move to the PHP League