This change makes btcd's getblock command match bitcoind's. Previously
the default verbosity was 0, which caused errors when using the
rpcclient library to connect to a bitcoind node - getblock would
unmarshall incorrectly since it didn't expect a verbosity=1 result when
it did not specify verbosity.
Due to differences in how getblock returns data based on the provided verbosity parameter, it's necessary
to have two separate return types based on verbosity. This necessitates a separate unmarshalling function
(represented throughout rpcclient/chain.go as Result.Receive()) to ensure that data is correctly unmarshalled
and returned to the user.
