PERF/BUG: Make the portfolio property call updated_portfolio.

Make the portfolio property on TradingAlgorithm call `updated_portfolio`
internally.  This prevents needless recomputation of the portfolio between
calls to `handle_data`, and also prevents issues where the portfolio object
could be unexpectedly modified in place in the body of a `handle_data` call.

Noteworthy finding in the course of investigating this bug:

If you modify a Python dictionary while iterating over it, the language will
only throw an exception if the size of the dictionary changes between loop
iterations; this means that you can do:
```
x = {1:1, 2:2, 3:3}
for k in x:
    old_val = x[k]
    del x[k]
    x[f(k)] = old_val
    print k
```
and you'll only get an error if f(k) is already a key in the dictionary.
This can lead to bizarre/nondeterministic behavior in the key iterator.

zipline/algorithm.py

@@ -601,13 +601,9 @@ class TradingAlgorithm(object):
 
     @property
     def portfolio(self):
-        # internally this will cause a refresh of the
-        # period performance calculations.
-        return self.perf_tracker.get_portfolio()
+        return self.updated_portfolio()
 
     def updated_portfolio(self):
-        # internally this will cause a refresh of the
-        # period performance calculations.
         if self.portfolio_needs_update:
             self._portfolio = self.perf_tracker.get_portfolio()
             self.portfolio_needs_update = False