MAINT: Convert per share cost to float during init.

The cost value should always be a float.
The per share calculation shouldn't be changed dramatically by being
a float, (except for potential rounding errors), but change to so
that PerShare and PerTrade are aligned.
 # Please enter the commit message for your changes. Lines starting

zipline/finance/commission.py

@@ -26,7 +26,7 @@ class PerShare(object):
         means three cents per share, which is a very conservative
         (quite high) for per share costs.
         """
-        self.cost = cost
+        self.cost = float(cost)
 
     def calculate(self, transaction):
         """