PROTON-194: CMake now builds proton-j-impl. Re-root jni thunking source code from bindings/java/jni/src to bindings/java/src. Various refactorings with the CMakeLists.txt including the extract function rebuild_jar which is shared between proton-jni and proton-j-impl.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/jni-binding@1439855 13f79535-47bb-0310-9956-ffa450edef68