Add support for using HttpContextBase, HttpRequestBase, HttpResponseBase...

As discussed in forums, here is the modification to support the enhancement.
....

These are equivalent classes to the non 'base' versions, but can be customized and used outside the asp.net pipeline.
This helps with unit testing and not using global HttpContext.Current.
Potentially the servicestack HttpResponseWrapper and HttpRequestWrapper classes could be just inherited from existing asp.net wrapper classes?