Wrap all date/locale function in new $dateUtil service

Currently the date and time pickers use `$dateParse` for formatting, `dateFilter` for formatting and `$locale` to determine the formats.

This works fine for most applications, but sometimes we want to use another library, e.g. [moment](http://momentjs.com/) to this work for us. This is true especially, if we already use moment for other components. Otherwise, we'd have to keep two sets of locale data, and there still might be some incompatibilities between the two.

This intermediate wrapper is needed to allow this customization. Check for example, [angular-strap with moment](http://plnkr.co/edit/inOk963yQlmxlxn6S9F3?p=preview) plunker.

Since the wrapper is placed in the `date-parser.js` and in the  `mgcrea.ngStrap.helpers.dateParser` module, this PC is completely backward compatible.