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

contrib/debian/copyright

@@ -37,7 +37,7 @@ Files: src/qt/res/icons/address-book.png, src/qt/res/icons/export.png,
        src/qt/res/icons/history.png, src/qt/res/icons/key.png,
        src/qt/res/icons/lock_*.png, src/qt/res/icons/overview.png,
        src/qt/res/icons/receive.png, src/qt/res/icons/send.png,
-       src/qt/res/icons/synced.png
+       src/qt/res/icons/synced.png, src/qt/res/icons/filesave.png
 Copyright: David Vignoni (david@icon-king.com)
            ICON KING - www.icon-king.com
 License: LGPL