Add a check to see if the node server is down or not

This PR is related to issue #40 (Node server shuts down and cannot restart). I haven't got a chance to actually look into why the node server goes down in some cases, but I feel it is definitely necessary for grinder to be able to restart the node server as long as the debugger is alive. If not, the debugging will just keep spawning the browser without any testcases.

Please note that I am not 100% familiar with grinder, so I'm not super confident if this implementation is appropriate or not. If you don't like it, please feel free to close.