Error if a session cannot be created.

A session cannot be created if the Set-Cookie header cannot be sent
to the webserver, IE after sendHeaders() has been invoked.

Opening the response OutputStream causes the headers to be sent. As
such a session cannot be created during the encode stage of a view.
Added an annotation to force a session to be created.

@RequireSession() can be used to annotate either all routes of
a router or individual routes, EG:

@RequireSession()
public clas MyRouter extends Router
{
    ...
}

@Get("/")
@RequireSession()
public void myRoute()
{
    ...
}