RuntimeStats: issue #44; allow for multiple timers to be used

- Change start_timer() and stop_timer() to accept a string argument,
  which is stored in a hash in RuntimeStats, so that multiple timers
  can be run at the same time
- Added time_value_difference() method to RuntimeStats
- Log debug how long it takes for each HTTP request
- Keep track of the total time spent preparing and making HTTP
  requests via LWP
- Push file IDs for unsuccessful API requests to a list so they can be
  printed when the script exits