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 
MethodLength is likely a good guideline, even in tests, and as this is a teaching site, the tests should be well written.  The question may be "At what point does testing classes length of methods become unwieldy?

That AbcSize talks about Assignment Branch and Conditions size.  If the tests are hard to read, then the tests serve less as documentation than they probably should.  But again, perhaps a higher threshold for tests than code when it comes to this metric as well.