WIP: change output to a JSON tree representation of XML

[squarebracket]

Need to rebase, and I haven't finished changing / adding all the tests, but this should give a good idea of what's coming.

Left to do:

• [ ] rebase to master
• [ ] end-to-end tests
• [ ] multiperiod gap tests

[gkatsev]

@squarebracket hey, do you remember what was left here and if it's worth for us taking a look and completing? Thanks.