Rubocop Sample config file and exempted cops in existing code

Take one or take both commits, but something to look at.

I will write something later in the README file to give hints on using Rubocop with exercise submissions.

re: #64 
I, personally, don't find method length and ABC to be useful metrics in test suites. I find that people tend to arbitrarily make up abstractions in order to reduce the line count and constrain assignments. For B and C I actually agree that too high a number probably indicates a difficult to read test, but I tend to be quite happy about longer methods and a fairly liberal use of temporary variables.

That said, I don't feel very strongly about it, and I suppose that there's more of a consensus in the Ruby community about short methods and few temporary variables, and I'm probably an outlier (or just an outsider).