Schema updates

I updated the schema to more closely match the v1.0 schema on Project Open data. It now allows a data.json file generated from inventory.data.gov to pass.

It still has limitations. It will not catch all format errors, especially for non-required fields. But at least it should allow "correct" data.json files (i.e. those generated from inventory.data.gov) to pass.

Fixes #2, Fixes #3, Fixes #7.