[breaking] Better treatment of ng-model.

When the element for a widget is a INPUT or SELECT (probably TEXTAREA too), the
Kendo directive was competing with AngularJS causing various issues (i.e. scope
updated twice: #135).  Previous workaround was to disable AngularJS own
input/select directives (7a1b7ef9cd837670ef089c2299ec259eeb9d51b1).  That
resulted in other breakage such as ng-dirty / ng-valid not working, or
confusing because the Angular scope got for instance a Date object, instead of a
string formatted according to options (#240).

To hopefully make everybody happy, I introduced a new option: k-ng-model.  Use
this one to bind the “logical” value, that is, whatever widget.value() returns.
Use ng-model to bind the “string” value -- $(element).val().

Closes #240.
Closes #227.
Document k-ng-model