Add Payu_In Integration

This is the Payu.in (https://www.payu.in/index) integration and tests as provided by them. 

I've done some minor code cleanup to get everything working correctly. One annoyance is they were depending on a PHP library in `web_service`. I've simply copy&pasted the library code into the `web_service` file for now - messy, but it works for now until they can remove the dependency.

@jduff