set up javadoc

[meisl]

how to deal with javadoc for individual plugins?

the root doc/api should probably contain only the doc for tc_java (without actual plugins' doc)

or maybe under root have both, doc/api and doc/api-all?

it's probably best to keep the generated javadoc (!) in the repo s.t. it can be easily browsed via https://htmlpreview.github.com?https://github.com/meisl/tc_java-NG/master/doc/api/index.html