remove jetty6 support, migrate needed jetty6 bits to GWTHostedMode, upda...

Jetty 6 was EOL'd in 2010 and there have been ~150 releases since then, so cleared out that support.

Also set the default support for Jetty to 7.2+, 8 and 9 since Jetty 7 and Jetty 8 are strictly in maintenance release mode locked in 7.6.x and 8.1.x respectively, and Jetty 9 is the current recommended stable release.

Tested this on 7.6.11, 8.1.11 and 9.0.3.

I documented setup for Jetty 9 (basically same for 7 and 8 as well) here: http://www.eclipse.org/jetty/documentation/current/framework-weld.html