Blade Postback & Model Binding Support

This commit adds postback (and XHR) support to Blade, enabling presenters to handle postback in a clean fashion by using ASP.NET MVC Model Binding to bind posted values to your viewmodel automatically.

MVC model validation attributes and validation controls are fully supported for Razor views, as are MVC HTML helpers (@Html.TextBoxFor(x=>x.StringField), for example). User Controls and Web Controls support validation, but they do not have ModelState to pass it around to so you'd need to manually parse the validation result in the presenter for those.

To support postback, if using a SitecorePresenter or SynthesisPresenter, simply override the HandlePostBackWithModelBinding method. The method is invoked if HTTP POST is requested, and model binding is automatically executed. Similar methods exist for XHR and non-automatic-modelbinding for both.

The postback handling occurs immediately after the OnLoad event just like web forms usually does for submit event handlers.