Update for latest btcjson.RawCmd changes.

This commit modifies all of the commands and notifications to work
properly with the latest btcjson.RawCmd Params field changes.

This new approach is superior to the old method of using a []interface{}
because it means that each parameter is now unmarshalled into the expected
concrete type directly instead of whatever it happens to be in the JSON.

Since it is now preferred to use full blown structs for individual
parameters, the RescanCmd type has been changed to use a new OutPoint
that can be used directly for marshalling and unmarshalling instead of a
*btcwire.OutPoint.

Also, all of the MarshalJSON functions now make use of the new
btcjson.NewRawCmd function rather than repeating the same common field
over and over and all of the MarshalJSON, UnmarshalJSON, and parseX
functions have been made more consistent.

ok @jrick