try to avoid kill -9 for stopping nodes

Using kill -9 on a node can leave it in a zombie process state, stuck in a
system call never to return. OS X Mavericks seems especially vunerable to
this problem. Only a reboot can clear out such zombies. Change
rt:brutal_kill/1 to try a normal kill -15 first, and set a 5 second timer
to perform a kill -9 if the normal kill doesn't work. Change
rtdev:stop_all/1 to first try to connect to the nodes to shut them down via
an init:stop/0 rpc, and if that fails attempt to stop them via "riak stop"
instead. Then, ps is used to check for any stragglers and those are killed
via kill -15, wait 5 seconds, kill -9.
Merge pull request #561 from basho/vinoski/nicer-kills

try to avoid kill -9 for stopping nodes