RFC : Added interrupt_workers

Patch provides a means to support  Ctrl-C for both remote as well as local workers as outlined in https://github.com/JuliaLang/julia/issues/3805 

```Base.interrupt_workers()``` must be called from the sigint handler as desired.

Interrupting workers has been implemented via callbacks to the ClusterManager. The system pid of the worker process is sent back as a response to the initial ```:join_pgrp``` message sent to the workers