Refactor WebpageRequest URL validation

- Modify WebpageRequest to use Net::HTTP.get_response() for validating
  URLs.
- Verify the temporary screenshot file exists after generating it from
  within the WebpageScreenshot.upload_screenshot method before
  continuing.
- Use FactoryGirl in Cucumber user step definitions for creating a
  new user.
- Change Cucumber's cassette directory to spec/fixtures/vcr_cassettes/
  in order to share with RSpec.
- Change User factory to use fake data.
- Re-enable all callbacks in specs.
- Remove Cucumber/Spork integration for now because otherwise Cucumber
  outputs a warning about how Spork is no longer supported as of 1.3.0.