Allow positve, float pitches as valid 'notes'.

Make micro-tuning and other absolute scale things simpler.

Ex:
   <instrument>.note('quarter', 'C#4, 888.2');

Invalid.
  <inst>.note('quarter', '-12312');  // no negatives
  <inst>.note('quarter', '0');  // must be positive
Merge pull request #12 from gregglind/floatpitches

Allow positive, float pitches as valid 'notes'.