feat($cordovaDatePicker): add promises

- add unit tests
- add promises

BREAKING CHNAGE:

Follow this to migrate:

Before:

```js
$cordovaDatePicker.show({
  date: new Date(),
  mode: 'date'
}, function(date) {
  alert(date);
});
```

After:

```js
$cordovaDatePicker.show({
  date: new Date(),
  mode: 'date'
}).then(function (date) {
    alert(date);
});
```

closes #246