Add support for multiple client implementations

[saden1]

See

• http://unirest.io/java.html
• http://square.github.io/okhttp/
• http://docs.spring.io/spring/docs/current/spring-framework-reference/htmlsingle/#rest-resttemplate
• https://developers.google.com/api-client-library/java/google-http-java-client/setup
• https://restlet.com/technical-resources/restlet-framework/guide/2.2/introduction/first-steps/first-client