http://www.codelord.net/2014/11/02/angularjs-1-dot-3-taste-async-validators/
http://stackoverflow.com/questions/20982751/custom-form-validation-directive-to-compare-two-fields
http://stackoverflow.com/questions/24227903/form-validation-required-one-of-many-in-a-group
http://stackoverflow.com/questions/24565719/simple-angularjs-date-input
