Allows the capital base of an algorithm to be more easily modified.

By having run() use a capital_base member of the algorithm to
create the trading environment, the capital base should now be
configurable in the instantiation of the algorithm.

e.g.:

```
algo = LowCapitalBaseAlgorithm(capital_base=1000.0):
```

zipline/algorithm.py

@@ -41,6 +41,8 @@ from zipline.gens.composites import (
 from zipline.gens.tradesimulation import TradeSimulationClient as tsc
 from zipline import MESSAGES
 
+DEFAULT_CAPITAL_BASE = float("1.0e5")
+
 
 class TradingAlgorithm(object):
     """Base class for trading algorithms. Inherit and overload
@@ -82,6 +84,9 @@ class TradingAlgorithm(object):
         self.slippage = VolumeShareSlippage()
         self.commission = PerShare()
 
+        # set the capital base
+        self.capital_base = kwargs.get('capital_base', DEFAULT_CAPITAL_BASE)
+
         # an algorithm subclass needs to set initialized to True
         # when it is fully initialized.
         self.initialized = False
@@ -175,7 +180,11 @@ class TradingAlgorithm(object):
 
             self.transforms.append(sf)
 
-        environment = create_trading_environment(start=start, end=end)
+        environment = create_trading_environment(
+            start=start,
+            end=end,
+            capital_base=self.capital_base
+        )
 
         # create transforms and zipline
         self.gen = self._create_generator(environment)