move configuration values to a config/system_pay.yml file; add railtie to load yml on startup; load form_helper automatically; update Readme accordingly
added all documented params; added a diagnose logic to decide on success/error; corrected Readme accordingly
prefer POST for return_mode
ignore nil instance variables
small error with strings and symbols in valid_signature
Merge pull request #3 from jcm/master

working and complete system_pay