Fixes JSON schema date regex patterns to match documentation

This responds to #209 as well as the comment on full 8601 dates - https://github.com/project-open-data/project-open-data.github.io/issues/153#issuecomment-29528070

Ultimately it might be more elegant to define a custom data format for JSON Schema, but for now we'll just have to manually use a regex to get the full 8601 date flexibility