Make datepair a jQuery plugin instead of a utility script

`datepair.js` now creates a jQuery plugin just like `.timepicker()`, so you can just call:

```javascript
$(selector).datepair();
```

It's also possible to supply options to the `datepicker()` and `timepicker()` widgets within the pair:

```javascript
$(selector).datepair({
  datepickerOpts: {
    'showButtonPanel': true
  },
  timepickerOpts: {
    'step': 5
  }
});
```

Also improved documentation in `index.html` for creating a datepair