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

src/rpcrawtransaction.cpp

@@ -304,6 +304,7 @@ Value listunspent(const Array& params, bool fHelp)
         }
         entry.push_back(Pair("amount",ValueFromAmount(nValue)));
         entry.push_back(Pair("confirmations",out.nDepth));
+        entry.push_back(Pair("spendable", out.fSpendable));
         results.push_back(entry);
     }