Make ServletValidator not use Jetty.

Jetty's just been too flaky, since this is only for user guidance, just use a loose parser.

http://gwt-code-reviews.appspot.com/284801/show

Fixes: 4760
Review by: bobv,tbroyer@gmail.com


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7823 8db76d5a-ed1c-0410-87a9-c151d255dfc7