This removes the many decodeXXXMessageSet() functions and replaces them
with a large map and a single generic decodeMessageSet() function. Also
change Responses to satisfy the Message interface as pointer types
(instead of the raw types), add the full set of top-level message sets
(though most of them still lack any message-parsing ability), adjust the
message set names to more closely mirror their OFX names, and fixup
tests and the command-line client to match the above changes.
