Extracted playing and pdf-viewing functions to player class
Extract options stuff to Settings class

This commit makes following side effects:
* Settings file format and filename was changed.
  (Python executable source -> json file)
* Now PDF dir option is gone. PDF file must in the same file as MIDI file,
  and must the same name as MIDI file.
  i.e. foobar.mid and foobar.pdf must be in the same directory.
bug fix.
arranged imports
Added license description to player.py and settings.py
Refactored
revised readme