fix(ngOptions): use watchCollection not deep watch of ngModel

Using a deep watch caused Angular to enter an infinite recursion in the
case that the model contains a circular reference.  Using `$watchCollection`
instead prevents this from happening.

This change means that we will not re-render the directive when there is
a change below the first level of properties on the model object. Making
such a change is a strange corner case that is unlikely to occur in practice
and the directive is not designed to support such a situation. The
documentation has been updated to clarify this behaviour.

This is not a breaking change since in 1.3.x this scenario did not work
at all. Compare 1.3.15: http://plnkr.co/edit/zsgnhflQ3M1ClUSrsne0?p=preview
to snapshot: http://plnkr.co/edit/hI48vBc0GscyYTYucJ0U?p=preview

Closes #11372
Closes #11653
Closes #11743