New methods, improved other sections of code.

 * Add check_melody, gen_melody methods.
 * Add half_prime_form method (via Music::NeoRiemannianTonnetz
   'normalize' code); used in check_melody.
 * Rework tests to use Test::Differences if available.
 * Eliminate Algorithm::Permute as can use Algorithm::Combinatorics for
   all such necessary operations.
 * Update subsets method to use Algorithm::Combinatorics; improve
   perldocs and add test for said method.
 * =head2 for method names for better visibility.
 * Inevitable speling fixes, tweaks, and other improvements.