Keep RSpec configuration between runs

RSpec resets the world and configuration objects after each run
(https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/runner.rb#L83). All custom helpers and matchers aren't accessible after reset.

So it's better to keep the RSpec's configuration defined in spec_helper.rb, because this
file is loaded only one (before the first run).

It should fix issues #13 and #19.