ENH: Remove dividends from the event stream.

Removes support for handling dividends as part of the algorithm
simulation stream, replacing it with an API in `TradingAlgorithm` for
supplying dividends as a DataFrame.

zipline/algorithm.py

@@ -665,6 +665,13 @@ class TradingAlgorithm(object):
         """
         self.blotter.transact = transact
 
+    def update_dividends(self, dividend_frame):
+        """
+        Set DataFrame used to process dividends.  DataFrame columns should
+        contain at least the entries in zp.DIVIDEND_FIELDS.
+        """
+        self.perf_tracker.update_dividends(dividend_frame)
+
     @api_method
     def set_slippage(self, slippage):
         if not isinstance(slippage, SlippageModel):