Add `setup_simplecov` helper method.

This provides a simple way to configure simplecov for all our projects.  I have a branch of rspec-expectations that uses this that I want to push later today so I'll probably go ahead and merge this in the near future.  Wanted to make a PR to make everyone aware of it, though.

/cc @xaviershay @JonRowe @soulcutter @samphippen @alindeman