Major version upgrade

1. Refactored HtmlWidgetBase/IHtmlWidget class/interface to minimize methods count and make it easy for extensions. IHtmlWidget interface now simply extends System.Web.IHtmlString interface. It's only necessary to implement ToHtmlString() method when extending from HtmlWidgetBase class.
2. HtmlWidgetBase.Write() method was renamed to Render().
3. Added new set of widgets along with unit tests :
- Gravatar [Avatar image URL, User profile URL]
- LiveJournal [Like Button, Repost Button]
- MailRu [Faces box, Groups box]
- SoundCloud [User profile icon]
4. Updated JSON.NET library version
5. Updated readme documentation