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.
