4.4.0: Developer console love

• Added trace marker (debug or error) to the server log files when
  the development console is shown.
  (@thekid)
• Merged PR #117: Catch errors and display them in development console.
  (@thekid)
• Fixed echo 0; (or 0.0, or "0") not triggering the development console
  (@thekid)
• Fixed trace data not appearing in logfile in development mode - @thekid
• Merged PR #115: Add web.Headers::qfactors(), which can be used when
  implementing content negotiation
  (@thekid)