[WIP] Severity

Add severities & payloadVersion.

Related Pull Requests:
https://github.com/bugsnag/bugsnag-website/pull/189
bugsnag/bugsnag-notifier-api#3
bugsnag/bugsnag-event-worker#59
bugsnag/bugsnag-ruby#96
bugsnag/bugsnag-php#39
bugsnag/bugsnag-cocoa#14
bugsnag/bugsnag-java#13
bugsnag/bugsnag-js#41
bugsnag/bugsnag-android#15