added flexible path prefix

At the moment gwt-pushstate reduces the path always to / as base e.g. if you use a tomcat Project with path "test" uses the url http://www.example.org:8080/test/Test.html it will be reduced to http://www.example.org:8080/ what doesn't work anymore. I've added a flexible pathprovider which simply returns the prefix, in the project you can add a Implementation of the PathProviderInterface and replace the interface in the project gwt.xml with the implementation.  I've added this at http://www.knightsoft-net.de/base16k/ (base16k it the prefix).