add optional tasks, complete assignment submission description

Author: cewing

assignments/session03/tasks.txt

@@ -29,4 +29,10 @@ Any instructions I need.
 
 The data you produce need not be pretty, or even particularly visible. In
 class we only produced a simple dictionary of values for each listing. Focus
-on getting data sources combined rather than on what the output looks like.
+on getting data sources combined rather than on what the output looks like.
+
+
+Optional Tasks:
+---------------
+
+Write unit tests supporting the functions of your mashup script.

source/presentations/session03.rst

@@ -1757,3 +1757,10 @@ To submit your homework:
 
 * Commit your changes to your fork of the repo in github, then open a pull
   request.
+
+
+Extra Credit
+------------
+
+Bonus points if you write unit tests for the elements of your mashup.  
+