Add funcs to Block API for wrapped transactions.

This commit adds two new functions to the Block API for working with the
recently added Tx type from the Block.

These new functions are named Tx and Transactions.  Tx returns a
transactions for the specified index as a Tx and also memoizes it so
subsequent calls are more efficient.  Transactions returns a slice of all
transactions in the Block wrapped in a Tx.

This is part of the ongoing transaction hash optimization effort
noted in conformal/btcd#25.