Revert "Disable payment protocol pending further assessment and potential rework."

This reverts commit 5967654730541a5598cb6005d7a05ef744efebc0.

Conflicts:

	src/qt/bitcoin.cpp
[Qt] ensure payment request network matches client network

- replaces checks in SendCoinsDialog::handlePaymentRequest() that belong
  to PaymentServer (normal URIs are special cased, as only an isValid
  check is done on BTC addresses)
- prevents the client to handle payment requests that do not match the
  clients network and shows an error instead (mainly a problem with
  drag&drop payment requests onto the client window)
- includes some small comment changes also
Revised payment request handling to use genesis block hash instead of network name.
Merge pull request #560 from rnicoll/1.7.2-dev-payment

Re-enable payment protocol