First class support for deploying R models

[dcrankshaw]

Motivation: Currently, Clipper supports model containers written in Python and Java/Scala. #195 extended the Python model container implementation to deploy R models and call them from Python containers. But this is pretty burdensome for developers and adds an unnecessary layer of indirection.

Feature: Clipper should support model containers directly implemented in R and out of the box support to deploy R models directly from an R program. There are two parts to this feature:

1. Implement a model container RPC client in C/C++ and wrap it with an R wrapper to create an R model container.
2. Create an R package that can be included in R programs with a Clipper.deploy_R_model method that will automatically serialize and deploy and R model, similar to the way we provide out of the box support for deploying Python functions.

[dcrankshaw]

Implemented in #261