This adds a python script to generate constants.go when `go generate` is
called, and updates the structs, tests, and command-line client to all
use the new named constants.
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.
This allows encoding/xml to properly comparison against nil to see if
they should be marshalled if the 'xml' tag contains ",omitempty" and for
users to test against nil to see if a field was present in the parsed
OFX.
This commit also fixes up cmd/ofx to use the new pointers.
