Add Monero as a payment method

[chill117]

This can be done in parts:

• [x] generatePaymentRequest(amount, cb) - Should call the callback function with an error (in case of failure) or with a string representing a valid payment request for monero. This string will be converted to a QR code and displayed to the customer.
• [x] checkPaymentReceived(paymentRequest, cb) - Should call the callback function with an error (in case of failure) or with a bolean (true or false) representing the state of the payment where true means received.
• [x] getExchangeRates(cb) - Should return an object like this:
  • { 'CURRENCY_CODE': 'RATE' } example:
  • { 'USD': '0.0012', 'EUR': '0.00093' }
• [x] Settings - See the settings array for the bitcoin payment method. This should probably include a "view key" and any other necessary user-level configurations for receiving monero payments.

See bitcoin payment method for inspiration.

[chill117]

I've created a new branch for monero, until it is ready to be merged into master.

[chill117]

• 77 resolves generating payment requests.

• The last remaining part is checking if a payment has been received

[chill117]

It should be possible to use the monerod RPC API to get current, unconfirmed transaction data:

• get_transaction_pool - to get unconfirmed transactions

To re-check old payment requests:

• getheight - to get the current block height of the monero blockchain. This will let us know from which block to search for a payment request's transaction.
• getblock - to get the transaction hashes included in an individual block.
• gettransactions - to get the data of one or more transactions.

During development, can use the publicly accessible nodes found here:

• https://moneroworld.com/#nodes

[chill117]

100 resolves checking for payment received

[chill117]

Done by #127