Issue #670 Clean up message initialization

- Severity should not be initialized to 0 (panic). It will now automatically
  default to 7 (debug) when unspecified.
- Leave all optional fields uninitialized so the don't waste space
  in the message if they are never used.

- http_listen_input is still hardcoded to 6 (info) we may want to
  revisit that in the future.
Merge pull request #696 from mozilla-services/issue_670

Issue #670 Clean up message initialization