Bug 1054178 - [Gaia][calendar] support 24 hour time format

we definitely need some special logic here to wrap the am/pm..

maybe instead of passing the `removeLeadingZero` we could add support for 2 new `[data-attributes]` like:

```html
<span class="display-hour" data-date="Thu Aug 28 2014 13:00:00 GMT-0300 (BRT)" data-l10n-date-format="week-hour-format12" data-l10n-pre-filter="amPm" data-l10n-post-filter="removeLeadingZero"></span>
```

where `[data-l10n-pre-filter]` is executed before the `localeFormat` call and `[data-l10n-post-filter]` is called after the `localeFormat`.

PS: this is just a suggestion.. maybe you have a better/simpler idea on how to handle these cases. The `[data-attributes]` are flexible but definitely increase the complexity a little, specially since so far we only need a couple different rules...