feat(ngClass): one can pass an $element variable into ngClass expression functions

When using directives like `ngClick`, `ngFocus`, `ngKeydown`, `ngMousemove` etc. AngularJS passes a special `$event` object to any function that basically is an event listener. 

One can then write in HTML
```html
ng-click="someHandler($event)"
```
and in javascript:
```javascript
$scope.someHandler = function(event){
    var element = $(event.currentTarget);
    // some other code
};
```
to get ahold of the HTML element (actually jQuery of jqLite object containing it) that the `ngClick` directive was applied to in the first place.

However in current version of AngularJS it is impossible to easily access that element using `ngClass` directive. This pull request makes it possible to pass a special `$element` object to any function in an `ngClass` expression so one can now write:

```html
ng-class="{'some-class': someCondition($element)}"
```

```javascript
$scope.someCondition = function(element){
    if(conditionsThatElementMustMeet(element))
        return true;
    else
        return false;
};
```

A real example might be a situation when one wants to apply a class to an element if and only if it's content is overflowing:

```css
#my-element{
    overflow: hidden;
    width: 50px;
    display: inline-block;
    white-space: nowrap;
    text-overflow: ellipsis;
}
```
```html
<div id="my-element" ng-class="{'it-is-overflowing': isOverflowing($element)}">
    {{modelBinding}}
</div>
```

```javascript
$scope.modelBinding = 'some string content'; // this could be user input
$scope.isOverflowing = function(element){
    // actual check whether the element's content is overflowing
    return element[0].scrollWidth > element[0].clientWidth;
};
```

Live code example here: http://jsfiddle.net/YAWGd/