Show a friendly message if an address is scanned

[Tbaut]

from #245

Users might be used to wallet applications that allow to scan a receiver's address to send funds to. Instead of showing a general error message, we can show a dedicated message if we detect that the QR code is in the form 0x... (20 bytes) or ethereurm:0x.. such as:

Ethereum address detected
You need to create a transaction using a software such as MyCrypto or Fether so that Parity Signer can sign it.

[hanwencheng]

After some research with UOS, and talked with YJ. I think the correct place is in the rawDataToU8A function in #273 , I will implement it after that one is finished.