Implement Servlet API 3.0

New versions of Pax Web, Equinox Web, Grizzly and other OSGI web servers will use version 3.0 of Servlet API. Sling Engine implementing only 2.5 will not run in these. Since Sling Engine implements API interfaces (ServletContext notably), it will have to implement new API methods and tighten some existing type signatures. Change is supposed to be backward-compatible (Sling Engine implementing 3.0 API can be used with 2.5 API as well).