add runpy django management command for lms

Runs a Python script within the edx platform's lms environment.

lms/djangoapps/courseware/management/commands/runpy.py

+from django.core.management.base import BaseCommand, CommandError
+
+
+class Command(BaseCommand):
+    args = "<script>"
+    help = "Run a Python script within the edx platform lms environment"
+
+    def handle(self, *args, **options):
+        if len(args) != 1:
+            raise CommandError("missing script argument")
+        script = args[0]
+        return execfile(script)