move wallet code to separate file

This introduces two new source files, keystore.cpp and wallet.cpp with
corresponding headers. Code is moved from main and db, in a preparation
for a follow-up commit which introduces the classes CWallet and CKeyStore.

src/keystore.h

@@ -0,0 +1,10 @@
+// Copyright (c) 2009-2011 Satoshi Nakamoto & Bitcoin developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file license.txt or http://www.opensource.org/licenses/mit-license.php.
+#ifndef BITCOIN_KEYSTORE_H
+#define BITCOIN_KEYSTORE_H
+
+bool AddKey(const CKey& key);
+std::vector<unsigned char> GenerateNewKey();
+
+#endif