Stock models

this is a minor refactor of the commtrack processing code, and a significant update of the xml parsing tests.

big thing is that we now save duplicately to the stock django models, which also generate the OTA restore payload.

four of the submission tests are failing due to what look to be real bugs in the code.

depends on https://github.com/dimagi/casexml/pull/141 (and the prior merged PRs into the casexml commtrack branch)