ngSelect directive rewritten to use ngModel.

hi, @pc035860!

Now we can benefit from all ngModel has (e.g. validations). See examples section for details.

ATTENTION! This PR breaks backward compatibility. You need to update your usage from `ng-select="expression"` to `ng-select ng-model="expression"`.

TODO: Update last 2 examples.

Also I want to notice that this PR includes https://github.com/pc035860/ngSelect/pull/4 so it maybe more convenient to use this comparision for review:  https://github.com/pluff/ngSelect/compare/feature%2Fobj_select_options...feature%2Fng_model?expand=1