allow rc.direct_view('all') to be lazily-evaluated

This means that the view will always apply to currently-attached
engines.

Previously, rc.direct_view('all') was equivalent to rc[:], which
is evaluated at construction.

The behavior of rc[:] is unchanged.

associated tests included

IPython/parallel/client/client.py

@@ -1024,6 +1024,8 class Client(HasTraits):
         targets: list,slice,int,etc. [default: use all engines]
             The subset of engines across which to load-balance
         """
+        if targets == 'all':
+            targets = None
         if targets is not None:
             targets = self._build_targets(targets)[1]
         return LoadBalancedView(client=self, socket=self._task_socket, targets=targets)
@@ -1041,7 +1043,9 class Client(HasTraits):
             The engines to use for the View
         """
         single = isinstance(targets, int)
-        targets = self._build_targets(targets)[1]
+        # allow 'all' to be lazily evaluated at each execution
+        if targets != 'all':
+            targets = self._build_targets(targets)[1]
         if single:
             targets = targets[0]
         return DirectView(client=self, socket=self._mux_socket, targets=targets)

IPython/parallel/tests/test_client.py

@@ -76,7 +76,16 class TestClient(ClusterTestCase):
         v = self.client.load_balanced_view(-1)
         self.assertEquals(v.targets, [self.client.ids[-1]])
         v = self.client.load_balanced_view('all')
-        self.assertEquals(v.targets, self.client.ids)
+        self.assertEquals(v.targets, None)
+    
+    def test_dview_targets(self):
+        """test load_balanced_view targets"""
+        v = self.client.direct_view()
+        self.assertEquals(v.targets, 'all')
+        v = self.client.direct_view('all')
+        self.assertEquals(v.targets, 'all')
+        v = self.client.direct_view(-1)
+        self.assertEquals(v.targets, self.client.ids[-1])
     
     def test_targets(self):
         """test various valid targets arguments"""