9/statusdb

This wraps up representing the status hierarchy in the data API.  There's no support for data migration or adding rows to the DB in the process code -- that's coming later.  I would like to get this out there and merged by the time I get to PyCon.

One thing I would like to change before merging is to accept either identifiers or numbers in URIs, rather than tagging one with `/number/`.  For example, a step could be
* `/builds/14/step/11` (not `/builds/14/step/number/11`)
* `/builds/14/step/shell_1`

but I can fix that up while this is in review.