ENH: Add IPython cell magic.

When zipline is imported it checks whether
it runs in the IPython notebook. If it does,
it registers a %%zipline magic that takes the
same arguments as the CLI with the addition of
a -o for specifying the output variable to store
the performance frame in.

The algo code in the cell is, as of yet, executed
in its own environment rather than that of the
IPython NB which is probably what we want.

Also adds cli option to save the perf dataframe
to a pickle file.

Also adds an IPython notebook buyapple example.

zipline/algorithm.py

@@ -126,6 +126,7 @@ class TradingAlgorithm(object):
         self.sources = []
 
         self._recorded_vars = {}
+        self.namespace = kwargs.get('namespace', {})
 
         self.logger = None
 
@@ -177,16 +178,15 @@ class TradingAlgorithm(object):
         self._analyze = None
 
         if self.algoscript is not None:
-            self.ns = {}
-            exec_(self.algoscript, self.ns)
-            self._initialize = self.ns.get('initialize', None)
-            if 'handle_data' not in self.ns:
+            exec_(self.algoscript, self.namespace)
+            self._initialize = self.namespace.get('initialize', None)
+            if 'handle_data' not in self.namespace:
                 raise ValueError('You must define a handle_data function.')
             else:
-                self._handle_data = self.ns['handle_data']
+                self._handle_data = self.namespace['handle_data']
 
             # Optional analyze function, gets called after run
-            self._analyze = self.ns.get('analyze', None)
+            self._analyze = self.namespace.get('analyze', None)
 
         elif kwargs.get('initialize', False) and kwargs.get('handle_data'):
             if self.algoscript is not None: