Issue #95 Allow the sandbox manager to control the sandbox usage limits

- The unit tests are just a configuration sanity check since an all up
  unit test is impractical.
- Manual testing was performed to verify the setting are actually enforced
  on running system.
Fix typo
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 #695 from mozilla-services/issue_95

Issue #95 Allow the sandbox manager to control the sandbox usage limits
Merge pull request #696 from mozilla-services/issue_670

Issue #670 Clean up message initialization
Merge branch 'dev' into issue_665

Conflicts:
	sandbox/plugins/sandbox_manager_filter.go