Adds the possibility to save a page as a working draft

In the UI it's now possible to uncheck the **Published** checkbox and thereby save the page as a working draft. Then if you edit the page again you can revert back to the published page and remove the working draft.

I have also introduced the concept of saving page drafts using the API.

```c#
var page = new MyPage();
session.Store(page); // Default RavenDB
session.Store(page, StoreAction.None); // Default RavenDB
session.Store(page, StoreAction.Save); // Save the page as a working draft
session.Store(page, StoreAction.Publish); // Publish a page, this will remove any working draft if available
session.Store(page, StoreAction.UnPublish); // Unpublish page
```