Added support for custom host

[emmanuelay]

This PR makes it possible to customize the host on each request.
Like this:

	apitest.New().
		Handler(handler).
		Host("demo.example.com").
		Get("/hello").
		Expect(t).
		Status(http.StatusOK).
		End()

I added this to facilitate testing a solution where the subdomain slug is used as a key to retrieve data needed in the handler.

[emmanuelay]

What do you think @steinfletcher ?