build an cheat-sheet about keystrokes on the fly
update README about cheat-sheet
move cheat-sheet builder in a tools directory

* use a tools directory instead of misc
* add Gemfile to handle dependency to run `buildcc`
ignore tools/.bundle directory
create misc folder to store cheat-sheet and logo in it

* require bundle/setup otherwise `bundle exec` is needed
Merge pull request #197 from tonini/cheat-sheet-generator

cheat-sheet with cabbage keystrokes.