Add transaction_context object to Computation

Ports over the changes made to master in ethereum#327.

Creates a BaseTransactionContext class and a child
FrontierTransactionContext class.
Removes properties which remain constant (gas_price, origin) from
Message and move them to the new TransactionContext class.
Generate a TransactionContext inside execute_transaction and attach it
to the Computation object. Pass this down into child Computations.
Begin reworking code which reads the (now-removed)
Message.{origin,gas_price}.

Update tests.

Errors from blockchain fixtures were being swallowed silently. Output
these along with a full stack trace to make debugging easier.

Author: lrettig

tests/json-fixtures/test_virtual_machine.py

@@ -192,8 +192,23 @@ def test_vm_fixtures(fixture, vm_class, computation_getter):
     # Update state_root manually
     vm.block.header.state_root = vm_state.state_root
 
-    computation = computation_getter(fixture, code, vm)
-
+    message = Message(
+        to=fixture['exec']['address'],
+        sender=fixture['exec']['caller'],
+        value=fixture['exec']['value'],
+        data=fixture['exec']['data'],
+        code=code,
+        gas=fixture['exec']['gas'],
+    )
+    transaction_context = BaseTransactionContext(
+        origin=fixture['exec']['origin'],
+        gas_price=fixture['exec']['gasPrice'],
+    )
+    computation = vm.state.get_computation(message, transaction_context).apply_computation(
+        vm.state,
+        message,
+        transaction_context,
+    )
     # Update state_root manually
     vm.block.header.state_root = computation.vm_state.state_root