Update task runner to allow multi-threading on OSX

Fixes bug #36. 
I think the issue is the original method "shares" the shell that is used by all the workers and therefore blocking other calls to that shell. Using subprocess.call with the shell=true is pretty much allowing each worker to spawn their own instance of a shell.

Interlace/lib/threader.py

@@ -1,4 +1,5 @@
 import threading
+import subprocess
 import os
 
 
@@ -20,7 +21,7 @@ class Worker(object):
 
     @staticmethod
     def run_task(task):
-        os.system(task)
+        subprocess.call(task, shell=true)
 
 
 class Pool(object):