Juwang-Http REST API

This request is to support the following  REST API for Http Server in REEF
	• Get List of evaluator Ids
	• Get Driver Info
	• Get Evaluator details such as Node Id, Node name, Internet Address and memory for given ids

It also contains schemas for http Responses 
 
It supports the following http requests:
	• Close - Allow clients to register their own Close Handler. When close request is received, it will trigger the Close Handler registered. If no registered Close handler, default one will be called. 
	• Kill - Allow client to send web request to kill Tasks, Evaluators and Driver. 
	
Resolve issues:  #868, #869, #870, #874

why not just `return `out.toString(...)` and get rid of that variable? also, why `.flush()`? I would assume that *at least* the `ByteArrayOutputStream` should flush before returning anything from `.toString()` call. (same for the encoder)