First version of Payum Redsys

WIP
===

First version of Redsys integration with payum. 

Actually capture part is done. You can connect to the bank gateway, do the operation there and receive the response from the bank 

Missed tests. I'll be adding them in the next days
I tried both variants, but haven't decided which one is the best. Let's add them to Api class, if you dont mind.