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.cpp

@@ -112,6 +112,12 @@ bool CWalletDB::WriteCScript(const uint160& hash, const CScript& redeemScript)
     return Write(std::make_pair(std::string("cscript"), hash), redeemScript, false);
 }
 
+bool CWalletDB::WriteWatchOnly(const CTxDestination &dest)
+{
+    nWalletDBUpdated++;
+    return Write(std::make_pair(std::string("watch"), CBitcoinAddress(dest).ToString()), '1');
+}
+
 bool CWalletDB::WriteBestBlock(const CBlockLocator& locator)
 {
     nWalletDBUpdated++;
@@ -404,6 +410,19 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
                     wss.fAnyUnordered = true;
             }
         }
+        else if (strType == "watch")
+        {
+            std::string strAddress;
+            ssKey >> strAddress;
+            char fYes;
+            ssValue >> fYes;
+            if (fYes == '1')
+                pwallet->LoadWatchOnly(CBitcoinAddress(strAddress).Get());
+
+            // Watch-only addresses have no birthday information for now,
+            // so set the wallet birthday to the beginning of time.
+            pwallet->nTimeFirstKey = 1;
+        }
         else if (strType == "key" || strType == "wkey")
         {
             CPubKey vchPubKey;