BUG: Algo no longer crashes after creating new history column.

Fixes an issue that caused a crash if a user assigned a new column into a
returned history DataFrame.  This occurred because we re-use DataFrames between
history() calls, so the new column caused an index-size mismatch on the next
attempted calculation.

Ideally we would fix this by in-place dropping the columns, but that isn't
supported in pandas 0.12.0, so instead we just return a complete copy of the
frame.  We should re-evaluate this implementation when we're on a more modern
pandas version.

tests/test_history.py

@@ -351,6 +351,7 @@ def initialize(context):
 
 def handle_data(context, data):
     prices = history(bar_count=2, frequency='1d', field='price')
+    prices['prices_times_two'] = prices[1] * 2
     context.last_prices = prices
 """.strip()