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