MAINT: Add a has_key method to BarData for compatibility.

BarData should, at least for the time being, be compatible with
existing algorithms that had worked against the prior usage of
an ndict as data, which provided `has_key`.

Of note, the Python language has deprecated `has_key` in favor
of using `in` and `__contains__`.

zipline/protocol.py

@@ -144,6 +144,13 @@ class BarData(object):
         else:
             return name in self.__dict__
 
+    def has_key(self, name):
+        """
+        DEPRECATED: __contains__ is preferred, but this method is for
+        compatibility with existing algorithms.
+        """
+        return name in self
+
     def __setitem__(self, name, value):
         self._data[name] = value