[WIP] Severity

Related Pull Requests:
bugsnag/bugsnag-website#189
bugsnag/bugsnag-notifier-api#3
bugsnag/bugsnag-event-worker#59
bugsnag/bugsnag-ruby#96
bugsnag/bugsnag-php#39
bugsnag/bugsnag-cocoa#14
https://github.com/bugsnag/bugsnag-java/pull/13
https://github.com/bugsnag/bugsnag-js/pull/41
https://github.com/bugsnag/bugsnag-python/pull/23