Update bip-0070.mediawiki

Slightly re-worded description of the signature field in PaymentRequest

The previous phrasing confused me and I had to check the Bitcoin Core source code to see what it was expecting for a signature.
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.
Added reference to RFC 2616 to BIP0072.
Added note about handling HTTP statuses which are neither error nor success.
Expanded and clarified wording on recovering from error states when sending payment to the merchant's server.
Update bip-0070.mediawiki

Change "zero-byte placeholder" to "empty string"
Loosened URL validity period on payment requests.
Update bip-0070.mediawiki

Revised final sentence of signature field description.
Minor grammatical & spelling corrections
Merge pull request #57 from e4xit/patch-1

Minor grammatical & spelling corrections
C# implementation

Github link to a C# implementation
Merge pull request #58 from NicolasDorier/patch-1

C# implementation
Validate a payment request is still valid prior to payment

Currently there exists the potential for a user to load a payment request into
their wallet which is valid at that time but its expiration lapses prior to
the user authorizing the payment. This could lead to an unnecessary customer
service interaction.
Merge pull request #41 from harding/master

Clarification of how to construct the PaymentRequest signature
Merge pull request #54 from rnicoll/bip0070-limits

BIP0070 limits and error handling
Merge pull request #60 from aalness/aalness_bip70

Validate a payment request is still valid prior to payment
Merge pull request #55 from rnicoll/bip0072-http

Add reference to RFC 2616 to BIP0072
Add reference implementation links

This BIP is really hard to implement because of the lack of test vectors and lack of documented  implementations. This will save time for implementers.