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 more than 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`. 
Any refactoring function can be called on pressing the appropriate key shown in the `cljr-key-mode` buffer. 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")