Add javadoc to Java class/method info.

This was a pretty straightforward addition. We nearly get Javadoc for free using the Java class/method info support. And `cider-javadoc` now works just like `cider-var-info` for Java symbols, so you can browse directly to class members.

It looks for docs on the classpath first, and then uses docs.oracle.com for core API classes.

Counterpart PR for `cider` is [#545](https://github.com/clojure-emacs/cider/pull/545).