Attribute binding improvements

Hi! I'm migrated from @derickbailey's Backbone.modelbinding to your project and want to get some code compatibility. If you don't like my changes in API, you probably would like some bugfixes in combineBindings for example. 

So I'll make three pull-requests, this one would not change the main API (except default bindings static method), rather than the following two.

This pull-request add possibility to use library like this:

    <div data-bind="hidden IsHidden">Splah</div>

    <form>
    <label>Name: <input name="Name"></label>
    </form>