Prevent transfers with a zero amount in the settle page

This workarounds a rounding issue caused by the (incorrect) usage of
floats for bill amounts.

This fixes #138

budget/models.py

@@ -61,9 +61,9 @@ def get_transactions_to_settle_bill(self):
         credits, debts, transactions = [],[],[]
         # Create lists of credits and debts
         for person in self.members:
-            if balance[person.id] > 0:
+            if round(balance[person.id], 2) > 0:
                 credits.append({"person": person, "balance": balance[person.id]})
-            elif balance[person.id] < 0:
+            elif round(balance[person.id], 2) < 0:
                 debts.append({"person": person, "balance": -balance[person.id]})
         # Try and find exact matches
         for credit in credits: