Some of the approximation features were in nascent form in previous releases. In
this release, we want to bring them closer to completion. Enhancements include:

* Add a Tonal::Ratio::Approximation::Set class to maintain the approximations
  and on which methods concerned with them can be located.
* Add a #by_superparticular strategy for finding approximate ratios.
* Rename the #approx method to #approximate, which reads better.

Additional changes include:

* Enhance the #superparticular method to allow assigning the superior part to
  the numerator (default) or denominator. This allows approaching target ratios
  from below as well as above. Also, add a factor argument, allowing finding
  superparticular ratios approaching any desired target ratio.
* Change the method name #normalize to #denominize. Normalize suggests extra
  undesired meaning. Denominize is a made up term, but its meaning is closer to
  what the method is actually doing.
* Extract the precision argument, located in method signatures, into a constant
  named PRECISION.
* Improve the readability of Tonal::Ratio#efficiency, by limiting its precision
  to 2 decimal places.
* Make available to_cents from instances of Tonal::Interval.
* Make ratio an alias to Tonal::Interval#interval.
* Add the ability to reduce ratios derived from prime divisions.
* Move the gem's version from the gemspec file, into a separate file, that will
  be changing more often.