used $parse service to prevent double $eval, render on event added

$scope.$eval is using $parse inside (http://stackoverflow.com/questions/15671471/angular-js-how-does-eval-work-and-why-is-it-different-from-vanilla-eval), so instead of calling $parse in $eval 2 times we store the result of $parse and re-use it. Added once-redraw-on attribute with event, which will force the directive to render the value.