Specify the json schema

```
We need a page/document that defines the json data format with enough
specificity that anyone (say a python programmer) can read it and then write
code that dumps out data caliper can read (or consumes data caliper produces).

Apparently there is no preeminent standard for how to write up a json schema
spec, or at least none Jesse was aware of, but we can just write up a doc in
whatever format, so long as it tells that programmer what he/she needs to know.

No specific urgency but I'm reluctant to start reaching out to other teams
about how they can share in the joy of our storage/presentment stuff until we
have this.
```

Original issue reported on code.google.com by `kevinb@google.com` on 6 Mar 2013 at 11:05