Improve Parameter Support

As discussed on the dev list [1], Sling's request parameter support should be extended and improved:

• Provide list of request parameters in the order specified in the request (the Servlet API does not define the order and servlet containers may or may not respect the order in the parameter map; but there is no official API to get the list of request parameters in the actual order as defined on the request)
• Support Servlet API 3.0 style multipart/form-data requests
• Make sure request parameters are uniformely supported on all servlet conainers with respect to character encoding and size restrictions.

As a corollary to Servlet API 3 support for request parameters, we might also want to extend the ServletContext API to implement the Servlet API 3 methods (even though, most will just throw)

[1] http://sling.markmail.org/thread/ysz5sxpb6wkuelzd