Add namespace browser

Extend CIDER documentation support to include a namespace browser.
`cider-browse-ns` will list all vars in a single ns.
`cider-browse-ns-all` will list all currently loaded namespaces.

Uses `cider-doc` to show documentation.

Defines a new major mode `cider-browse-ms-mode` which behaves similarly to dired. It supports `n`, `p` and `^` in addition to `return` and mouse clicks.

Questions:

* Do you want any default keybindings for the interactive functions? I'm using `C-c M-b` and `C-c M-B` in my personal config.
* This PR contains some simple clojure forms embedded in the elisp. Is that ok or should I move them to the cider-nrepl project?