Fix #1041 - YUI Editor should be able to work without iframe

This PR introduces a new feature in YUI Editor - it will be able to work in two modes now:
1. Using iframe (the old approach)
2. As an inline (so called "HTML5") editor

For that purpose, the architecture of the Editor has been changed a bit. It now has two containers - the old one iframe and a new one, called "Content Editable".
Both are plugins and extend Plugin.Base.

Following and old conversation with @davglass, and to achieve backward compatibility, if there is no container specified, EditorBase will load the old iframe.

Here is an example how YUI Editor could be transformed to an inline editor:

````javascript
    var myEditor = new Y.EditorBase({
        content: '<p>Hello world. I\'m an inline editor. No iframe here!</p>',
        plugins: [
            Y.Plugin.ContentEditable
        ]
    });

    myEditor.render('#editor');
````

As you see, the only difference is that we plug a plugin - Y.Plugin.ContentEditable. If we don't plug it, we will have an editor with an iframe.

There are no API changes, so people should have 100% backward compatibility.

To prove that, let's use a Toolbar from YUI Gallery, for example [Itsa EditorToolbar](http://yuilibrary.com/gallery/show/itsatoolbar) by @ItsAsbreuk:

````javascript
     var myEditor = new Y.EditorBase({
        content: '<p>Hello world. I\'m an inline editor. No iframe here!</p>',
        plugins: [
            Y.Plugin.ContentEditable,
            {
                fn: Y.Plugin.ITSAToolbar,
                cfg: {
                    srcNode: '#toolbar'
                }
            }
        ]
    });

    myEditor.render('#editor');
````

The toolbar works flawlessly directly loaded from Yahoo! CDN.