Tool to compare dashboards across environments

Assumes staging and preview currently, pass the slug as an argument and
it will request both and diff them.
Return errors as per JSON API suggestion

See http://jsonapi.org/format/#errors
Merge pull request #230 from alphagov/feature/json-api-errors

Return errors as per JSON API suggestion
Merge pull request #254 from alphagov/add-compare-dashboards-tool

Tool to compare dashboards across environments