Add support for watch-only addresses

Changes:

• Add Add/Have WatchOnly methods to CKeyStore, and implementations in CBasicKeyStore.
• Add similar methods to CWallet, and support entries for it in CWalletDB.
• Make IsMine in script/wallet return a new enum 'isminetype', rather than a boolean. This allows distinguishing between spendable and unspendable coins.
• Add a field fSpendable to COutput (GetAvailableCoins' return type).
• Mark watchonly coins in listunspent as 'watchonly': true.
• Add 'watchonly' to validateaddress, suppressing script/pubkey/... in this case.

Based on a patch by Eric Lombrozo.

Conflicts:
src/qt/walletmodel.cpp
src/rpcserver.cpp
src/wallet.cpp