Add support OkHttp library

[lenguyenthanh]

As OkHttp has many advantages and there a lot of Android applications already use OkHttp. Should this library supports OkHttp as a plugin or uses it directly?

[yoavst]

In order to do that, you need to create a Client that uses OkHttp.

Here is the default client: HttpClient

I never used OkHttp.

[lenguyenthanh]

So should we create a library call fuel-okhttp in this repository?

[kittinunf]

I think it should be separated. You could open up with your repo that supports OkHttp. I am happy to link it in readme of this repo.