Allow getblock result to be string or BlockResult.

The getblock command has recently added a verbose flag which alters the
output.  The previous code added a new field "Hex" to the BlockResult, but
this is not consistent with the origin getblock RPC call which returns a
string when verbose is false and the BlockResult JSON object when it is
true.

This commit corrects that by first removing the Hex field from the
BlockResult and second allowing the result for getblock to be either form.