Remove Tx model

There was a split between a transaction request and an actual
transaction. This was kind of annoying because transactions were one-way
only, while transaction requests were two-way - which is what I believe
most transactions will be using. Tx model has been removed and the
responsibilities of it are covered by TxRequest. It simplifies
everything surrounding transactions, since we have only one model to
deal with instead of two.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

trading/tests/test_models.py

@@ -1,38 +1,8 @@
 from django.test import TestCase
-from trading.models import User, Commodity, Tx, TxRequest, BalanceError
+from trading.models import User, Commodity, TxRequest, BalanceError
 
 
 class TestTransactions(TestCase):
-    def test_user_balances(self):
-        # Create users
-        user1 = User.objects.create_user(username="test1", email="test1@test.test")
-        user2 = User.objects.create_user(username="test2", email="test2@test.test")
-
-        # Create test commodity
-        commodity = Commodity(created_by=user1, in_circulation=1000, name="commodity")
-        commodity.save()
-
-        # Confirm balances
-        self.assertEqual(user1.balances(), {commodity: 1000})
-        self.assertEqual(user1.balance_of(commodity), 1000)
-        self.assertEqual(user2.balances(), {})
-        self.assertEqual(user2.balance_of(commodity), 0)
-
-        # Create transaction
-        Tx(source=user1, dest=user2, amount=100, commodity=commodity).save()
-
-        # Confirm balances
-        self.assertEqual(user1.balances(), {commodity: 900})
-        self.assertEqual(user2.balances(), {commodity: 100})
-
-        # Ensure we can't trade when there's missing balance
-        with self.assertRaises(BalanceError):
-            Tx(source=user2, dest=user1, amount=1000, commodity=commodity).save()
-
-        # Confirm balances
-        self.assertEqual(user1.balances(), {commodity: 900})
-        self.assertEqual(user2.balances(), {commodity: 100})
-
     def test_tx_requests(self):
         # Create users
         user1 = User.objects.create_user(username="test1", email="test1@test.test")