Add HaveClass verification extension method

[YevgeniyShunevych]

public static TOwner HaveClass<TComponent, TOwner>(this IUIComponentVerificationProvider<TComponent, TOwner> should, params string[] classNames)
    where TComponent : UIComponent<TOwner>
    where TOwner : PageObject<TOwner>;

Provides shorter syntax to verify class.

Instead of:

Component.Attributes.Class.Should.Contain("some-class")

use:

Component.Should.HaveClass("some-class")