Session object model

Add the basic object model for Session.

Also add support for removing features from a collection.
Notes from offline: I tried `Abandon()` on vCurrent ASP.NET and found that it does not delete the cookie. Abandon behaves the same as Clear, except that it waits until the end of the request before clearing the content. The next request will still have the same sessionId unless you specify a web-config option to regenerate it. Regardless, once you call abandon there's no way to establish a new session on the same request.

In short, there's no back-compat motivation to support Abandon. We may consider it as a future enhancement.