Adding PRG and WebMethods support for WebForms

This is an initial commit that shows how we could support WebMethods and the PRG pattern.

In the MVC3 sample project an additional action has been defined on the HomeController that shows that we can store a post (get in this case, but that doesn't matter) as well as the get that happens after the redirect

Compile locally and run the MVC3 app and make sure Glimpse is loaded show history tab) and then access the following uri: http://localhost:33333/Home/PRGTest, the initial request to the PRGTest action will be made visible on the history tab (you can see the SQL executed for instance), as well as the last get request (which has always been visible)

I also added a WebMethodsAndPostRedirectGetTests.aspx page to the Wingtip project which has 3 buttons of which their corresponding click postback handlers do one of the following actions:
 
- Response.Redirect("/")
- Server.Transfer("About.aspx")
- Throw an exception

I also added the WebMethods sample given in #596 so show that it also works and is being tracked when clicking on the get time button

You'll see if you launch the Wingtip project that there is an additional "WebMethods and PRG Tests" menu item, which will bring you to the buttons mentioned above

But I think we need to check what the possible issues could be, asking tabs for data later in the pipeline.
The proposed solution is backward compatible, in that sense that the abstract class `HttpApplicationBase` has an additional virtual member that does nothing except when overridden. (in v2 this could become an abstract member like it is the case for the other members)

ps: why is` HttpApplicationBase` an abstract class without implementation and not an interface?