JSON does not allow the use of single quotation, so do not encourage it.
Use Faraday in WebsiteAgent and make HTTP backend library selectable.

Typhoeus is still suggested as default, but you could choose 'em_http',
or 'net_http' if you have a problem with it.
Add a `user_agent` option to WebsiteAgent.
Merge pull request #264 from knu/website_agent-faraday

 Add support for using alternative HTTP backends via Faraday