Add master volume property and put dynamics on note

Updates
+ Add master volume property in main constructor
+ Add dynamics property in note

e.g.)
```js
var music = beeplay({volume: 0.5})
  .play('C4', 1, 'pp');
  .play('D4', 1, 'p')
  .play('E4', 1, 'mp')
  .play('F4', 1) // or 'm'
  .play('G4', 1, 'mf')
  .play('A4', 1, 'f')
  .play('B4', 1, 'ff');
```