Use asynchronous event queue, HttpServletRequest.getContextPath() return null with tomcat

1. ZK 6.5.1.1 works fine with tomcat;
2. ZK 6.0.2 ~ ZK 6.5.0 doesn't work with tomcat, but if run with "jetty", it works fine.

the null pointer exception is cause by
String ctxpath = ((HttpServletRequest)request).getContextPath();
in ClassWebResource.java