fix(datepicker): ensure format string is initialized

Delayed format string initialization using attrs.$observer causes errors with datepickerPopup, because parser is constructed too early. This error was triggered by using datepickerPopup on a initially hidden tab-pane.

TypeError: Cannot read property 'split' of undefined
    at createParser (http://localhost/js/vendor.min.js:1339:33)
    at parse (http://localhost/js/vendor.min.js:1373:35)
    at parseDate (http://localhost/js/vendor.min.js:2066:33)
    at ngModel.$render (http://localhost/js/vendor.min.js:2105:22)