Add payment processor for FRED backend

[stinovlas]

Add new payment processor (see #12) for FRED backend. Communication with FRED backend will be done through CORBA interface. Method process_payment will be implemented as follows:

• processor asks backend whether it recognizes the variable symbol
  • if no, processor returns False
  • if yes, processor saves invoice details and continue
• processor asks backend to create invoice; in return, it will recieve total amount of credits that should be added to registrar account
• processor asks backend to add designated amount of credits to registrar account in appropriate zone

[stinovlas]

Moved to separate project fred-pain.