Removed some NullReferenceException, added an alternative way of configuring the objects and added fluent configurators.

[jgauffin]

I got NullReferenceException for different properties like the InputStream. Those shouldn't have to be configured by should be setup per default. This PR initializes all properties with relevant default values for request/response/context.

As an alternative to the Get extension method there is now a property for httpcontext/request/response called Control which exposes all properties as setters/getters. The syntax becomes a little bit cleaner (request.Control.HttpMethod = "POST"). All existing unit tests do of course pass with flying colors (i.e. I've tried to make everything backwards compatible).

Finally I've introduced a fluent syntax for using this library:

// simple context
var httpContext = builder
    .Post("http://localhost/?A=1")
    .WithForm(new { FirstName = "Jonas", LastName = "Gauffin", UserName = "jgauffin" })
    .RespondWith(403)
    .Build();

// configuring the session
var httpContext = builder
    .UsingSession(new {UserId = 10})
    .Build();

// setting principal
var principal ? new GenericPrincipal(new GenericIdentity("Arne"), new []{"Admin"});
var httpContext = builder
    .UsePrincipal(principal)
    .Build();