Moves the current coin selection algorithm out of SelectCoinsMinConf
and puts it in coinselection.{cpp,h}. The new function, KnapsackSolver,
instead of taking a vector of COutputs, will take a vector of CInputCoins
that is prepared by SelectCoinsMinConf.
Changes CInputCoin to coinselection and to use CTransactionRef in
order to avoid a circular dependency. Also moves other coin selection
specific variables out of wallet.h to coinselectoin.h
