[Fix #626] Enable `cider-test-run-test` on any Clojure buffer.

Previously this command was only enabled in the report buffer and on tests that had previous failures/errors. This requires `clojure-mode` [#238](https://github.com/clojure-emacs/clojure-mode/pull/238).

Also, update context menus and key binding documentation.
Btw, `C-c letter` keybindings are reserved for users and cannot be used in mode maps. We should alter this.