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/rpcclient.cpp

@@ -72,6 +72,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "lockunspent", 0 },
     { "lockunspent", 1 },
     { "importprivkey", 2 },
+    { "importaddress", 2 },
     { "verifychain", 0 },
     { "verifychain", 1 },
     { "keypoolrefill", 0 },