Add "disabledDates" functionality

Add `disabledDates` config option
Add `setDisabledDates` method
Update documentation

```
new Pikaday({
  field: el,
  minDate: new Date(2014, 0, 20),
  maxDate: new Date(2014, 0, 31),
  disabledDates: [new Date(2014, 0, 25), new Date(2014, 0, 26)]
})
```