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.h

@@ -74,6 +74,7 @@ public:
     bool ChangeWalletPassphrase(const SecureString& strOldWalletPassphrase, const SecureString& strNewWalletPassphrase);
     bool EncryptWallet(const SecureString& strWalletPassphrase);
 
+    void MarkDirty();
     bool AddToWallet(const CWalletTx& wtxIn);
     bool AddToWalletIfInvolvingMe(const CTransaction& tx, const CBlock* pblock, bool fUpdate = false, bool fFindBlock = false);
     bool EraseFromWallet(uint256 hash);