Bug/required multiple select

[jcolemorrison]

Closes the following:

#850

and

#914

The fix adds a custom validator to multiple selects that enables the required tag. Custom validators are not "my" own customized way of doing things, they are the official angular way of writing validators as outlined in the Custom Validators section here https://docs.angularjs.org/guide/forms

The problem was that required checks the ngModel's viewModel, which when empty on a ui-select-multiple is an empty array...which is technically not empty. Now it checks properly without interfering with the standard required validator.

Also fixed a test bug where $item.name wasn't being used in the multiple example.

[guidoffm]

I have found this problem: When I also have the ng-change attribute pointing to a function and when I remove the last element from the multiple selection then the model is set to undefined.

[user378230]

I believe #1492 fixes this and has been merged, so closing. If you disagree please let us know.