improve web fragment support

when configuring ZK with a servlet 3 web.xml the web-fragment.xml in zk.jar/META-INF/web-fragment.xml automatically registers ZK's servlet with default settings / mappings.

Whenever the defaults are unwanted, things get more complicated and sometimes container specific. Ignoring a specific web-fragment.xml is container specific or instead all "wanted" web-fragments need to be listed.

removing or providing the web-fragment.xml in a separate dependency would help in that matter allowing for more flexible configuration. The benefits of the default config are only initial convenience for trivial tests. While the disadvantages make more complex configurations difficult.