add pry-byebug to Gemfile
- kill clear_failures
- make time_window an initialization parameter
- inject Time.now to help tests
- make succes_count, failure_count and error_rate public
- better testing for ratio
- add public method clear_open_flag to be able to try to clse the
  circuit before sleep window is finished.
- add test to check ratio calculation after sleep window make
  the circuit to open.
remove pry-byebug from Gemfile
Merge pull request #2 from yammer/fix_rate_calculation

Fix rate calculation