#319 fix for lock selections

Adding ability to lock selections.

In a multi-select, if you set the `locked` attribute in your options it will lock the selection.

   //-> options = [ {  id: 123, name: "sally", locked: true } ]
    <ui-select ng-model="model.people">
        <ui-select-match>{{$select.selected.name}}</ui-select-match>
        <ui-select-choices  repeat="value.id as value in options | filter: $select.search">
            <div ng-bind-html="value.name | highlight: $select.search"></div>
        </ui-select-choices>
    </ui-select>