Add ability to provide your own HTTP Client

[Hontaro]

In Unirest you had to provide your own HTTPClient via Unirest.setHttpClient(httpClient). This functionality has been lost with the RestClient.
The current ClientBuilder is missing customization options provided by the HttpClientBuilder such as setting your own SSL context.

[joshgontijo]

Yeah, it makes sense, the http-client API surface area is massive. The RestClient builder provides a very simplified configuration of the most used methods of it. I decided to stick with a minimal approach, and expand it on request basis. Will add SSL context for the builder and option to provide your own HttpClient on 1.6.1 release.