First part of the Servlet3 web-fragment support.

First part of the Servlet3 web-fragment support.
This patch inserts a stage into the web.xml processing. web.xml files are now parsed and loaded into an instance of WebXml and then that WebXml instance is used to configure the context. This will allow the subsequent addition of parsing and merging web-fragment.xml files. The hooks are in place to add this functionality but it has not yet been written.
With the patch applied, the Servlet 2.5 TCK and JSP 2.1 TCK pass with the exception of the Servlet version number checks.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@816951 13f79535-47bb-0310-9956-ffa450edef68