Provide a riak-shell command to stop running instance [JIRA: RTS-1330]

[augb]

When an instance of riak-shell has been started on a local machine, and an attempt to start another instance of riak-shell on the same machine is made, one gets the following error message:

An instance of riak-shell is already running

This is fine, except there is no way to stop the shell gracefully, in some circumstances. It would be helpful to have a riak-shell command to stop an already running instance of riak-shell on the local machine.

Please see this question on Stack Overflow: [1] for more details for why this may matter beyond basic convenience.

[1] http://stackoverflow.com/questions/38897124/how-do-i-get-back-to-the-running-instance-of-riak-shell