Cleans up README and provides idea for level 4

Renames README to markdown extension (.md) and ruby warrior code to ruby extension (.rb) for automatic syntax highlighting in tools like GitHub and most text editors.
Refactors using `space.enemy?`

I wish there were more docs in https://www.bloc.io/ruby-warrior
Merge pull request #1 from edward/ideas-for-level-4

Cleans up README and provides idea for level 4