Various improvements.

- Moved units table to module level.
- Renamed format_time() to format_duration().
- Added optional output file param to ElapsedTimer constructor.
- Refactored __enter__ and __exit__ implementation into other methods,
  including a new start() and stop().
- Added properties for a local enable, elapsed time, and a property to
  get the elapsed time as a datetime.timedelta object.
Filled in readme.