calc checksum on provided fields, not all fields

• checksum is calculated on provided fields, not all fields. Thanks @edelooff

fix python3 issue

0.3.1

fix for python 3

getBasicPaymentURL

• implemented getBasicPaymentURL mehtod which generates a URL for basic payment mode, w/o rest api call

validate_postback

• added validate_postback(data) method to verify postback checksum

first release

0.1.2

cast merchant id to str, bump version