Download pmd-build-tools-plugin JAR file with all dependencies

This small subproject regroup most of the stuff needed to build pmd. As some java code has been produced to reduce the use of scripts when generating the website, the build also requires a small jar, which is generated by this pom. IMPORTANT NOTE: the pmd-build.jar does not need to be recompile if you modify any ant or xslt files. You can alter the files that are in the xslt directory and directly uses the appropriate ant or java task. pmd-build should be recompiled and package (and its version incrememented) only if you modify the java code.