Re-configure VCR, use Typhoeus for URL validation

- Use Typhoeus for URL validation
- Use the same cassette names for Cucumber as RSpec
- Re-configure VCR to hook into WebMock and to ignore
  http://not.a.valid.url
- Don't stub requests to http://www.unknown_response.com now that
  Typhoeus is being used for URL validation
- Use http://www.yahoo.com for test URL in controller