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/test/wallet_tests.cpp

@@ -46,7 +46,7 @@ static void add_coin(int64_t nValue, int nAge = 6*24, bool fIsFromMe = false, in
         wtx->fDebitCached = true;
         wtx->nDebitCached = 1;
     }
-    COutput output(wtx, nInput, nAge);
+    COutput output(wtx, nInput, nAge, true);
     vCoins.push_back(output);
 }