Changed CLI to utilize Launchy to open the graph

This should patch the error issue I raised a few days ago.  I added a new Spec for the CLI class, updated the code to make the test pass, and added the new dependency on Launchy into the gemspec file.

This is my first pull request, so I've likely made all manner of mistakes.  Please let me know if there's anything I need to correct.
I might rewrite this as 

```ruby
cli.should_receive(:open_in_browser)
begin
  cli.execute
rescue SystemExit => systemexit
  systemexit.success?.should be_true
end
```