Make protected methods private

Child classes are allowed to call private methods of parent class.
Protected methods can called by another instances from same class or
its children.

See http://stackoverflow.com/a/1565640/3108885.
Handle space key properly

KeyHelper::KEY_CODES convert key having its key code as 32 to `:space`,
not `:' '`.
Use recommended format for comment annotations

See https://github.com/bbatsov/ruby-style-guide#comment-annotations.
Clean up comments
Update documentation
Merge pull request #126 from yous/patch-private

Make protected methods private
Merge pull request #127 from yous/fix-space

Handle space key properly
Merge pull request #128 from yous/patch-documentation

Update documentation