Added some items to TODO list.

TODO

@@ -114,3 +114,7 @@ Division of 0 by 1 reduces to 0.
 
  - Parser:
    + add unit tests for operator associativity (derivatives/integrals).
+
+ - Add sum rule for integrals.
+
+ - [x^3/3 + c]_2^4  ->  [x^3/3]_2^4  # Remove all 'constant' variables before substitution