Improve DateHelper

As stated in #383, let's improve the DateHelper (or whatever name it may have in 5.7 series).
If and once it's accepted we should update the core to use these new functions.
Currently we have:
```PHP
echo $dateHelper->date($format, $timestamp);
```
With these new functions we'll have
```PHP
echo $dateHelper->formatDate($anyDateTimeRepresentation);
// or
echo $dateHelper->formatTime($anyDateTimeRepresentation);
// or
echo $dateHelper->formatDateTime($anyDateTimeRepresentation);
```

As a bonus, we have also the new functions `describeInterval`, `getDeltaDays`, `formatPrettyDate`, `formatPrettyDateTime`, `formatCustom` and `getJQueryUIDatePickerFormat` (see function comments for more details).