Add REST Client API

Currently, there is only protobuf-based TCP protocol for client. It works well.

However, even though protobuf itself is very portable, our protobuf-based client protocol is not portable. This is because protocol buffer does not provide RPC implementation and it just provides RPC stub and marshalling method.

The main objective of this issue is to implement RESTful API for client. This technique is very widely used in many applications. REST does not require any heavy dependency. It is expected that many third-party client API can be easily implemented with REST client API.