CWallet class

• A new class CKeyStore manages private keys, and script.cpp depends on access to CKeyStore.
• A new class CWallet extends CKeyStore, and contains all former wallet-specific globals; CWallet depends on script.cpp, not the other way around.
• Wallet-specific functions in CTransaction/CTxIn/CTxOut (GetDebit, GetCredit, GetChange, IsMine, IsFromMe), are moved to CWallet, taking their former 'this' argument as an explicit parameter
• CWalletTx objects know which CWallet they belong to, for convenience, so they have their own direct (and caching) GetDebit/... functions.
• Some code was moved from CWalletDB to CWallet, such as handling of reserve keys.
• Main.cpp keeps a set of all 'registered' wallets, which should be informed about updates to the block chain, and does not have any notion about any 'main' wallet. Function in main.cpp that require a wallet (such as GenerateCoins), take an explicit CWallet* argument.
• The actual CWallet instance used by the application is defined in init.cpp as "CWallet* pwalletMain". rpc.cpp and ui.cpp use this variable.
• Functions in main.cpp and db.cpp that are not used by other modules are marked static.
• The code for handling the 'submitorder' message is removed, as it not really compatible with the idea that a node is independent from the wallet(s) connected to it, and obsolete anyway.