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.
fix help prompt
Merge pull request #2957 from Rav3nPL/patch-1

fix help prompt
fix the typo fix in linearize.py
Merge pull request #2959 from jmcorgan/fix-typo

fix the typo fix in linearize.py
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
Don't warn about forks while fIsInitialDownload
Merge pull request #2944 from gavinandresen/acceptheader

Set Accept: application/bitcoin-paymentrequest header
Add protobuf 2.5.0 download to gitian release process
Qt 4.8.3 r2 in gitian and release process

Also needed to add `unzip` package for the added unzip command for
OpenSSL in the Qt build.
Make settxfee clear that units are btc/kb.