Merge remote-tracking branch 'origin/master' into vik/add-htsql

Conflicts:
	anserv/courseware_old/models.py
	anserv/datalib/courseware/models.py
	anserv/datalib/student/models.py
	anserv/mitx_libraries/courseware/models.py
	anserv/mitx_libraries/student/models.py
	anserv/modules/course_stats/course_stats.py
	anserv/modules/tasks.py
	anserv/modules/user_stats/user_stats.py
	anserv/settings.py
	anserv/student_old/models.py
	requirements.txt

README.md

@@ -24,6 +24,10 @@ tracking architecture, a read-replica of the main database, as well
 as, in the future, course definition database, and read-replicas of
 auxilliary databases, such as discussion forums. 
 
+A block diagram of the overall system is: 
+
+![System structure](system_structure.png)
+
 Each module in the analytics framework has its own Mongo database. In
 addition, in the near future, it should have read-only access to the
 DBs associated with other modules.