This allows for ofxgo to be used to create well-formatted OFX from poor
OFX, or even be used to generate OFX from other formats for easier
importing into financial management software.
Test this functionality by adding "round trip" testing to all existing
tests - ensure that responses' content is the same after a round trip of
marshalling and unmarshalling them.
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.
