Added Hamming Distance (aimacode#340)

In learning.py, I added the Hamming Distance metric.

Author: antmarakis

learning.py

@@ -35,6 +35,9 @@ def manhattan_distance(predictions, targets):
 def mean_boolean_error(predictions, targets):
     return mean(int(p != t) for p, t in zip(predictions, targets))
 
+def hamming_distance(predictions, targets):
+    return sum(p != t for p, t in zip(predictions, targets))
+
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