Added shopper_sig to Adyen::Form

Adyen offers to send shopper info to the Adyen Form flow.

Send the shopper information with the fields (at least):

    shopper.firstName
    shopper.lastName
    shopper.socialSecurityNumber (that's the CPF/CNPJ)

shopperSig: shopper.firstName + shopper.infix + shopper.lastName + shopper.gender + shopper.dateOfBirthDayOfMonth + shopper.dateOfBirthMonth + shopper.dateOfBirthYear + shopper.telephoneNumber
(Note that you can send only shopper.firstName and shopper.lastName if you like. Do NOT include shopperSocialSecurityNumber in the shopperSig!)
Added tests for shopper Adyen::Form attributes.
Merge pull request #75 from joost/patch-2

Added shopper_sig to Adyen::Form