Added ta max text directive for #117

I tried to make the max text value observable. Well that part was easy but when the value changes I could not get the validation to re-rerun. Inside the observable callback i tried calling ``ctrl.$setViewValue(validator(ctrl.$modelValue))`` but $modelValue kept returning as ``NaN``. Weird. 

#117 

I tried to write some tests but.. failed. I've not done js testing before.