added support for description to all form fields

solves GH-52. 

Note on the [`aria-describedby`](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Techniques/Using_the_aria-describedby_attribute#Related_ARIA_techniques.C2.A0) attributes: There is a [a11y plugin](https://github.com/angular/angular.js/pull/8342) in the making, which would remove this attribute if description is empty, but currently I haven't found a (good) solution to this problem.