Added 'validationFn' attribute to directive to enable consumers to define routines that extend beyond the
'required' scenario.

User-defined routines can be provided on their scope as a funtion which returns eiether a bool, or an object
with 'isValid' bool property and a 'message' property describing the reason for success or failure. Supporting
implementation added to js/source/core.js, and the one-liner in the directive definition.  Test code in
robust.js and index.html was also added.
Grunt build for latest build/robusticus.js
Merge pull request #5 from dariusriggins/validationFn

Validation fn