Adding path validation

Doing 2 checks:
- Check if we have an absolute path
- Check if the file exists

Requires 'pathname' gem
Remove false as we use 'unless'
Add file in the error message
Merge pull request #391 from electrical/Path_validator

Adding path validation
- include the format that failed in date filter errors.
- don't include a stack trace anymore
- Add doc generator support for path validation (#391)
- style
- make json parse failures info, not warns
- fix variable names
- fix performance problem in the lumberjack input
  The cause is that Event[]= is now quite slow. This mainly impacts
  lumberjack since it called it so many times, but there's a simple
  workaround for lumberjack.

  As for improving the performance of Event[]=, that'll be a task
  for another day.
- computers. :(