improvements to invoice

[PiRK]

• show formatted invoice (at payment time)
• option to export invoice as pdf
• more optional fields
  • invoice creation date (automatically filled, with an edit option)
  • invoice payment date (automatically filled, with an edit option)
  • name and address of payer
  • name and address of payee
• show fiat amount for information when user enters a XEC amount
• add a button to fill the "fixed rate" from the selected API