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/script.h

@@ -5,6 +5,7 @@
 #define H_BITCOIN_SCRIPT
 
 #include "base58.h"
+#include "keystore.h"
 
 #include <string>
 #include <vector>