Add RESTclient layer

It has a nice interface for doing work with APIs and whatnot

> This is a tool to manually explore and test HTTP REST webservices. Runs queries from a plain-text query sheet, > displays results as a pretty-printed XML, JSON and even images.
 
Checkoutt [Emacs Rocks!](http://emacsrocks.com/e15.html)
[Package Link](https://github.com/pashky/restclient.el)
to be consistent with the key bindings with a REPL [1][2] I prefer those bindings:
- "ms": send and stay in window
- "mS": send and switch window
- "mr": send raw and stay in window
-"mR": send raw and switch window

[1] https://github.com/syl20bnr/spacemacs/tree/master/contrib/lang/ess
[2] https://github.com/syl20bnr/spacemacs/tree/master/contrib/lang/python
See how lower letters does not change the focus whereas capital one change the focus to the REPL