Implement NFC payments

[mislavmatijevic]

Currently, only QR code and normal code payments are accepted.

However, a real cool thing would be to allow NFC payments.

Acceptance Criteria

Condition	Description
Given	User1 is logged in successfully as a seller and on products/packages page.
When	User1 selects products for sale and proceeds to initiate invoice generation.
Then	User1 is offered "Use NFC for payment" option.
And
Given	User2 is logged in successfully as a buyer and on BUY menu.
When	User2 selects NFC Payment as payment method.
Then	User1's and User2's phones touch and NFC payment is initiated, User2 is immediately presented with INVOICE DETAILS screen.

This method should work the same as current QR code method, including allowing the manual code entry option that is currently enabled for QR code scanning (a failsafe option).

Problem with implementing this is - testing, since two test devices are needed. Either 2 emulators (good-bye 20GBs) or 2 real-life devices (which would require approaching another fellow Android-user human and asking them to borrow their phone - arghh!).

Of course, this method would be enabled only on Android and iOS devices.