New app form validation

This adds form validation based on the validation method in App.js. Any errors returned from a call to `validate` mark the invalid fields in the form.

* Validation logic for the "cmd" attribute was taken from a commit by @mohitsoni: https://github.com/mohitsoni/marathon/commit/f61f2337290ec9921acb7f090582f0481855fa43
* Addresses the problem raised in pull request #137