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
Doc from the gateway only says that a valid operation will produce a Reply from the bank between 0 and 99. But yes. Could be constants. Should them be added to Api or maybe to a dedicated Constant class like offline payment?