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.
Update bip-0070.mediawiki

Change "zero-byte placeholder" to "empty string"
Update bip-0070.mediawiki

Revised final sentence of signature field description.
Merge pull request #41 from harding/master

Clarification of how to construct the PaymentRequest signature