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
qt: Handle address purpose in incremental updates

Correctly use the purpose of addresses that are added after the start
of the client. Addresses with purpose "refund" and "change" should not
be visible in the GUI. This is now handled correctly.
qt: Display txfee in first sendCoinsDialog message box

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Bitcoin-Qt: allow to differentiate Qt log entries from core

- prepend "Bitcoin-Qt" in front of debug.log entries, which come from Qt
- move DebugMessageHandler installation upwards to the event handler
  installation, which fits much better
Merge pull request #2963 from Diapolo/log-qt

Bitcoin-Qt: allow to differentiate Qt log entries from core
Merge pull request #2951 from laanwj/2013_08_addressbook_purpose

[Qt] Handle address purpose in incremental updates
Fix typo in a comment: it's base58, not base48.
Merge pull request #2967 from tstranex/master

Fix typo in a comment: it's base58, not base48.
Merge pull request #2958 from laanwj/2013_08_txfee2

[Qt] Display txfee in first sendCoinsDialog message box
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