Adds an init to protocol.Event to enable setting of initial values.

So that an Event can use an initial dict to set all values,
instead of needing to set initial values one by one.

i.e. enables:

```
foo = Event({'bar': 1, 'baz': 2})
```

in favor of:

```
foo = Event()
foo.bar = 1
foo.baz = 2
```

zipline/protocol.py

@@ -31,6 +31,10 @@ DATASOURCE_TYPE = Enum(
 
 class Event(object):
 
+    def __init__(self, initial_values=None):
+        if initial_values:
+            self.__dict__ = initial_values
+
     def __getitem__(self, name):
         return getattr(self, name)