[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.
Merge pull request #628 from jeffvalk/master

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