Use 'info' middleware for Javadoc.

Accompanies `cider-nrepl` [#47](https://github.com/clojure-emacs/cider-nrepl/pull/47)

This makes middleware update functional, but leaves an open question on Javadoc user experience -- should we use an internal or external browser? I'd prefer the former, but this PR doesn't change the default. I should note that local Javadoc URLs (on the classpath) may be returned as jar/zip URLs. This might need to be changed, depending on how we open them.