Move to pure OkHttp request handling.

[serj-lotutovici]

Since starting from Android KitKat the HttpUrlConnection implementation is using the OkHttp one, it makes sense to use OkHttp directly for network requests to avoid an additional implementation layer.

The feature must consist of:

• Removing unnecessary RequestExecutor abstraction. All request manipulations must go through the request client.
• Possibility to return raw responses for additional error and custom response handling
• Consider possibility of removing the custom Request implementations
• XingController must allow initialisation with a custom (or semi-custom) OkHttpClient

[serj-lotutovici]

With CallSpec we are pretty much done here.