A measured change

Added

• Duration.with_measure, which yields and returns an array containing its
  evaluated return value and its Duration.

Changed

• Break Duration and Instant into their own files.
• Rename Monotime::VERSION to Monotime::MONOTIME_VERSION to reduce
  potential for collision if the module is included.
• Update to bundler 2.0.
• Rework README.md. Includes fix for issue #1 (added a "See Also" section).