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/walletdb.h

@@ -7,6 +7,7 @@
 
 #include "db.h"
 #include "key.h"
+#include "keystore.h"
 
 #include <list>
 #include <stdint.h>
@@ -93,6 +94,8 @@ public:
 
     bool WriteCScript(const uint160& hash, const CScript& redeemScript);
 
+    bool WriteWatchOnly(const CTxDestination &dest);
+
     bool WriteBestBlock(const CBlockLocator& locator);
     bool ReadBestBlock(CBlockLocator& locator);