Fix getbalance discrepency

Two changes:

Use IsConfirmed() instead of IsFinal(), so 'getbalance "*" 0' uses the same
'is this output spendable' criteria as 'getbalance'. Fixes issue #172.

And a tiny refactor to CWallet::GetBalance() (redundant call to IsFinal -- IsConfirmed
calls IsFinal).
getbalance with no arguments and 'getbalance "*" 0' could return different different results,

src/wallet.cpp

@@ -899,7 +899,7 @@ int64 CWallet::GetBalance() const
         for (map<uint256, CWalletTx>::const_iterator it = mapWallet.begin(); it != mapWallet.end(); ++it)
         {
             const CWalletTx* pcoin = &(*it).second;
-            if (pcoin->IsFinal() && pcoin->IsConfirmed())
+            if (pcoin->IsConfirmed())
                 nTotal += pcoin->GetAvailableCredit();
         }
     }