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/keystore.cpp

@@ -59,3 +59,15 @@ bool CBasicKeyStore::GetCScript(const CScriptID &hash, CScript& redeemScriptOut)
     return false;
 }
 
+bool CBasicKeyStore::AddWatchOnly(const CTxDestination &dest)
+{
+    LOCK(cs_KeyStore);
+    setWatchOnly.insert(dest);
+    return true;
+}
+
+bool CBasicKeyStore::HaveWatchOnly(const CTxDestination &dest) const
+{
+    LOCK(cs_KeyStore);
+    return setWatchOnly.count(dest) > 0;
+}