Add security warning disclaimer in the "Payment request" text

As suggested by @harding on https://github.com/saivann/bitcoin.org/issues/86#issuecomment-41602838

This pull request adds a single warning disclaimer at the end of the "Payment request" text and removes all duplicate ones later in each subsections.

I re-used @harding text with a few tweaks so this text also applies to QR codes and clearly state the real nature of the risk with MITM attacks ( attackers replacing the Bitcoin address ).