Implemented interactive help for refactoring

Closes #33

- Provides a simple interactive help window for key bindings in
  clj-refactor.el minor mode.
- Added tests.
- Added key binding to README.

This feature is based on [magit-key-mode.el](https://github.com/magit/magit/blob/master/magit-key-mode.el). 

Use `?` in cljr minor mode to show the interactive help buffer. It looks like this right now.
As far as the string messages go, I'm willing to change them if needed.

![alt text](https://raw.github.com/darth10/imgs/master/clj-refactor.el.interactive-refactoring.1.png "cljr-key-mode")

Now, on pressing `a`, the user is shows refactoring key bindings that start with `a`. Also, `?` will invoke `describe-key` for the key bindings that are shown.

![alt text](https://raw.github.com/darth10/imgs/master/clj-refactor.el.interactive-refactoring.2.png "cljr-key-mode")