Developer-private settings files

Developers can have private settings files by creating
lms/envs/private.py or cms/envs/private.py.  They are imported
at the end of dev.py.  Note that they won't be imported if you
are using one of the other dev*.py variants.

cms/envs/dev.py

@@ -165,3 +165,11 @@ MITX_FEATURES['ENABLE_SERVICE_STATUS'] = True
 
 # segment-io key for dev
 SEGMENT_IO_KEY = 'mty8edrrsg'
+
+
+#####################################################################
+# Lastly, see if the developer has any local overrides.
+try:
+    from .private import *
+except ImportError:
+    pass