Added 'Backup Wallet' menu option

- icon from the LGPL Nuvola set (like the tick) - http://www.icon-king.com/projects/nuvola/
- include 'boost/version.hpp' in db.cpp so that the overwrite version of copy can be used
- catch exceptions in BackupWallet (e.g. filesystem_error thrown when trying to overwrite without the overwrite flag set)
- include db.h in walletmodel.cpp for BackupWallet function
- updated doc/assets-attribution.txt and contrib/debian/copyright with copyright info for new icon
increase BitcoinAddressValidator::MaxAddressLength from 34 to 35

needed for sending BIP16 transactions on test network through GUI
fix default suffixes in save dialog in GNOME, make it more clear that PNG is used (solves #833)
Bugfix: do not create CAddress for invalid accepts
Several shutdown-related fixes

* do not let vnThreadsRunning[1] go negative
* do not perform locking operations while vnThreadsRunning[1] is decreased
* check vnThreadsRunning[1] at exit
Fix compilation warning.
Make bitcoin-qt static-link libqrcode and add it for debian pkgs.
Properly include $*_LIB_PATH in makefile.unix
Merge pull request #828 from sipa/fixwalletlock

Fix wallet locking locking
Merge pull request #842 from sipa/checkaf

Do not create CAddress for invalid accepts
Merge pull request #844 from sipa/shutdownfix

Several shutdown-related fixes
Symbolic names for threads

Introduce an enum threadId, and use symbolic indices when accessing
vnThreadsRunning.
Merge pull request #843 from TheBlueMatt/qrcodedoc

Fix one compile-time warning and fix libqrcode linking in releases
Fix issue #848 : broken mining on testnet
Fix #846.  Allow negative options such as "nolisten=1" in bitcoin.conf as well as on the command line.
fix typo in CNetAddr::IsRFC4843()  (fixes #850)
don't allow -daemon in bitcoin-qt (changes only #defines)
Restructure IPC URL handling (fixes #851)
Hide window from taskbar when "minimize to tray" active by making window into Tool window
On windows, show message box with help, as there is no stderr (fixes #702)