Add a ServletContext configuration parameter for enabling/disabling automatic session creation in WeldListener/ServletLifecycle

As noted by Heiko here:

http://github.com/weld/core/pull/10

, the ServletListener breaks in some versions of Jetty, since getSession() is unable to create a new Session.

However, completely disabling new session creation for the listener changes the way ServletListener behaves, in that the first request hitting a newly started server may not have a sessioncontext/requestcontext/conversationcontext assigned to it.

While this can work fine in general, I think that we need some ability to finely tune this, by adding a ServletContext parameter which would control this behaviour (allow new session creation by default, but allow users to turn this off)