feat(date): new dateFormatter helper

Factor out the date formatting handling into a new service `dateFormatter`.
While the default implementation uses angular's dateFilter and $locale, custom
implementations may use other libraries, e.g. [moment](http://momentjs.com/).

BREAKING CHANGE: If you include angular-strap files individually (from the `dist/modules`
directory) than `dist/modules/date-formatter.js` is now required to use the date/time
pickers.