Make Vesta URL config explicit and add optional logger config

[jacksonh]

Vesta recently changed their URL configs for the sandbox, so the JSON and JSONP URLs have different domains. Since there are now 4 possible URL combos, its easier to just allow explicitly setting the URLs instead of having the underlying system join paths.

A typical setup might look something like this now:

VSafe.configure do |config|
  config.sandbox_url = 'https://sanboxdomain.com/GatewayProxy/Service'
  config.sandbox_jsonp_url  = 'https://sandbox-otherdomain.com/GatewayProxyJSON/Service'
  config.production_url = ''https://production.com/GatewayProxyJSON/Service'
  config.production_jsonp_url = ''https://production.com/GatewayProxyJSON/Service'
  config.logger = Rails.logger
end

I setup all the URLs based on the previous defaults, so if Vesta hasn't changed your URLs this should just work for you.

This PR also adds the option to pass a logger down to HTTParty. Doing config.logger = Rails.logger will set the HTTParty logger when making requests and give nice request/response logging.

[ngan]

Hey @jacksonh this looks good, can you update the README as well?

[jacksonh]

Thanks! Will do!

[jacksonh]

@ngan alright, updated the docs. Let me know how that looks.