Add bibliography and sources to 2 strategies.

Adding them to Grudger and TitForTat.

axelrod/strategies/grudger.py

@@ -3,7 +3,16 @@
 C, D = Actions.C, Actions.D
 
 class Grudger(Player):
-    """A player starts by cooperating however will defect if at any point the opponent has defected."""
+    """
+    A player starts by cooperating however will defect if at any point the
+    opponent has defected.
+
+    Names:
+
+    - Friedman's strategy: [Axelrod1980]_
+    - Grudger: [Li2011]_
+    - Grim: [Berg2015]_
+    """
 
     name = 'Grudger'
     classifier = {

axelrod/strategies/titfortat.py

@@ -12,6 +12,11 @@ class TitForTat(Player):
     Note that the code for this strategy is written in a fairly verbose
     way. This is done so that it can serve as an example strategy for
     those who might be new to Python.
+
+    Names
+
+    - Rapoport's strategy: [Axelrod1980]_
+    - TitForTat: [Axelrod1980]_
     """
 
     # These are various properties for the strategy

docs/reference/bibliography.rst

@@ -0,0 +1,5 @@
+Bibliography
+============
+.. [Axelrod1980] Axelrod, R. (1980). Effective Choice in the Prisoner’s Dilemma. Journal of Conflict Resolution, 24(1), 3–25.
+.. [Berg2015] Berg, P. Van Den, & Weissing, F. J. (2015). The importance of mechanisms for the evolution of cooperation. Proceedings of the Royal Society B-Biological Sciences, 282.
+.. [Li2011] Li, J., Hingston, P., Member, S., & Kendall, G. (2011). Engineering Design of Strategies for Winning Iterated Prisoner ’ s Dilemma Competitions, 3(4), 348–360.

docs/reference/index.rst

@@ -12,4 +12,5 @@ Contents:
    play_contexts.rst
    overview_of_strategies.rst
    all_strategies.rst
+   bibliography.rst
    glossary.rst