MAINT: Re-enable algorithms which omit initialize method.

Though defining the `initialize` method may end up being explicitly
required, in the meantime prevent existing algorithms from crashing
by providing a noop function when `initialize` is not defined.

zipline/algorithm.py

@@ -160,6 +160,8 @@ class TradingAlgorithm(object):
         # functions.
         self.algoscript = kwargs.pop('script', None)
 
+        self._initialize = None
+
         if self.algoscript is not None:
             self.ns = {}
             exec_(self.algoscript, self.ns)
@@ -179,6 +181,9 @@ class TradingAlgorithm(object):
             self._initialize = kwargs.pop('initialize')
             self._handle_data = kwargs.pop('handle_data')
 
+        if self._initialize is None:
+            self._initialize = lambda x: None
+
         # an algorithm subclass needs to set initialized to True when
         # it is fully initialized.
         self.initialized = False