[Qt] payment request work - part 1

I'm going to once more dig into our payment request code to better polish and add some more things to it. This is just the start and I intend to create some small pulls and not a big one, so it's easier to review and merge. After I get an ACK for this I will add a cleanup commit to this one, which I leave out for now.

For reference the BIPs:
https://github.com/bitcoin/bips/blob/master/bip-0070.mediawiki
https://github.com/bitcoin/bips/blob/master/bip-0071.mediawiki

@laanwj Perhaps you can help me getting this done smoothly :)?
@fanquake After the initial work here, do you want to participate by converting the paymentserver and paymentrequestplus files into the doxygen format after I got ACKs :)?
I wonder if there is a way to tell Qt to download only a maximum size of request. The problem with handling it in Finished() is that part of the DoS already has been done: we downloaded all the data into memory.