Replaced example domain name with example.com.
Added file size limits for Payment and PaymentACK messages.
Added note about handling multiple copies of a Payment message, to ensure resend is safe
in case of a transport layer failure.
Expanded and clarified wording on recovering from error states when sending payment to the merchant's server.
Loosened URL validity period on payment requests.
Merge pull request #54 from rnicoll/bip0070-limits

BIP0070 limits and error handling