Removed maturity check from CWalletTx::GetCredit and
CWalletTx::GetAvailableCredit (for consistency) and created new
function CWalletTx::IsMature() to be used on the outside when needed.
This change was needed to allow the QT client to list the correct
amount on an immature proof of stake transaction.

Also added immature stake coins to CWallet::GetImmatureBalance() so
the QT client will get a correct total.