complete implementation of the revenue sharing model

See the leading comment for a detailed explanation.

Signed-off-by: Loïc Dachary <loic@dachary.org>

@@ -3,0 +69,4 @@
+    members = [["a", 10, 5], ["b", 450, 25], ["c", 0, 0]]
+    payable, payments = share_income(members)
+    assert payable == [["a", 10, 5], ["b", 20, 25]]
+    assert payments == {"a": [("b", 5)]}

@@ -3,0 +35,4 @@
+# For instance in the if the 'members' argument is [['isabelle', 200, 10]], the
+# shares will be [['isabelle', 10, 10]] where the 200 expense was reduced to a
+# share of 10 because there only is 10 income available.
+#

@@ -3,0 +50,4 @@
+#    'marie': [('isabelle', 200), ('john', 50)],
+#    'lucie': [('john', 10)],
+#  }
+#