Key import and export

Introduces two new RPC calls:
* dumpprivkey: retrieve the private key corresponding to an address
* importprivkey: add a private key to your wallet

The private key format is analoguous to the address format. It is
a 51-character base58-encoded string, that includes a version number
and a checksum.

Includes patch by mhanne:
* add optional account parameter for importprivkey, if omitted use default

src/wallet.cpp

@@ -224,6 +224,15 @@ void CWallet::WalletUpdateSpent(const CTransaction &tx)
     }
 }
 
+void CWallet::MarkDirty()
+{
+    CRITICAL_BLOCK(cs_wallet)
+    {
+        BOOST_FOREACH(PAIRTYPE(const uint256, CWalletTx)& item, mapWallet)
+            item.second.MarkDirty();
+    }
+}
+
 bool CWallet::AddToWallet(const CWalletTx& wtxIn)
 {
     uint256 hash = wtxIn.GetHash();
@@ -1445,6 +1454,7 @@ void CWallet::GetAllReserveAddresses(set<CBitcoinAddress>& setAddress)
     CWalletDB walletdb(strWalletFile);
 
     CRITICAL_BLOCK(cs_main)
+    CRITICAL_BLOCK(cs_wallet)
     BOOST_FOREACH(const int64& id, setKeyPool)
     {
         CKeyPool keypool;