Add documentation viewer.

This uses the new Java `info` backend from `cider-nrepl` [#70][70] to display Javadoc -- along with the usual Clojure docstrings -- in a new `cider-doc` mode. There are more possibilities here, but to get an idea, type:

`java.lang.String`
`javax.net.ssl.SSLEngine`

in any `cider-mode` buffer, then `C-c C-d` on those symbols.

[70]: https://github.com/clojure-emacs/cider-nrepl/pull/70