Pull request to prevent constant redefinition in tests

During testing, RSpec warns of a constant being redefined, probably due to adopting before(:each) in pull request #193.

This (new) request reverts back to `before(:all)`. But it replaces the conflicting `let` statements with instance variables, per this insightful [comment](http://stackoverflow.com/questions/5359558/when-to-use-rspec-let#comment14970850_9994464) by principal RSpec maintainer Myron Marston.

All tests pass.