feat(ngToggle): add directive to show and hide two DOM elements

This directive provide a complement for ng-show and ng-hide directives
and change the visibility of two DOM elements depending on an expression.
ngToggle is a frequently used operation and can be implemented like the
following example:

Example:

    <ng:toggle on="exp">
        <div id="A"></div>
        <div id="B"></div>
    </ng:toggle>

If exp is evaluated to true then only #A is set to visible, otherwise
only #B is set to visible.

Closes [#228](https://github.com/angular/angular.js/issues/228)