Add support for complex union types.

Our `make_schema_strict` logic didn't properly handle these types.  We have to set `additionalProperties = false` on the individual type definitions within the union rather than on the union type itself.
Update to latest json-schema version.

It's supposed to be threadsafe, among other things.
Only allow defined formats.

We've been accidentally using invalid formats like "timestamp" in some places, so this helps ensure we use formats that will actually have a validation effect.
Merge pull request #16 from seomoz/fixes

Fixes