Fixed signature version 3 calculation
Updated getUrl and calculateSignature methods

getUrl
calculateSignature is now called correctly at line 97

calculateSignature
Constants SIGNATURE_VERSION_1 and SIGNATURE_VERSION_3 are now accessed correctly at lines 123 and 156 respectively
Merge pull request #1 from paymentwall-dev/calculateSignature

Updated getUrl and calculateSignature methods
Merge pull request #1 from paymentwall-dev/master

Fixed signature version 3 calculation and issues wigh calculateSignature method