Fixes javadocs build

JavaDocs build was broken for me (lubuntu Virtualbox VM 14.10 both jdk8 and jdk7, using "gradlew", tried on tag as well): it was complaining about missing Pegdown doclet (because of unspecified docletpath).

I'm no Gradle evangelist but I tried to factor out at least repositories in a "allprojects" section; I think it could be possible to factor out even more (JavaDocs links seem to need some fixes as well).