Set Accept: application/bitcoin-paymentrequest header

Suggestion from BitPay: set an Accept: HTTP header when
fetching payment requests / payment ACKs.
RPC: add getrawchangeaddress, for raw transaction change destinations
Merge pull request #2948 from jgarzik/changeaddr

RPC: add getrawchangeaddress, for raw transaction change destinations
Merge pull request #2944 from gavinandresen/acceptheader

Set Accept: application/bitcoin-paymentrequest header