Restructured all the modules

The core module was becoming too large.
By splitting it in  cluster, monitor and tasks it becomes more manageable. It also reflects the documentation structure.

django_q/management/commands/qcluster.py

@@ -1,9 +1,11 @@
 from django.core.management.base import BaseCommand
-from django_q.core import Cluster
+from django.utils.translation import ugettext as _
+from django_q.cluster import Cluster
 
 
 class Command(BaseCommand):
-    help = "Starts a Django Q Cluster."
+    # Translators: help text for qcluster management command
+    help = _("Starts a Django Q Cluster.")
 
     def handle(self, *args, **options):
         q = Cluster()