Support gradle for jpf-core and extensions.

JPF is a model checking tool for java applications. JPF-core is the core structure of JPF. The build for jpf-core has been moved from ant (upto java 8 support version)to gradle. The current JPF-core version doesn’t have java 11 support i.e jpf-core is not portable for java 11. Jpf extensions have not moved from ant to gradle yet. This is because of the potential breaking changes from its previous versions. One such major breaking change in java 11 is “bootstrap methods”. The goal of this project is to fix gradle support for java 11, to update the extension template,provide the widely used jpf-extensions with gradle support.