push cell magic to the head of the transformer line

I'm not 100% sure this is the right fix (ping @takluyver for review),
since it is unclear what the physical/logical distinction is,

but this does allow the cell magic transform to preempt all other transforms.

closes  #3604

IPython/core/inputsplitter.py

@@ -486,7 +486,9 b' class IPythonInputSplitter(InputSplitter):'
         if physical_line_transforms is not None:
             self.physical_line_transforms = physical_line_transforms
         else:
-            self.physical_line_transforms = [leading_indent(),
+            self.physical_line_transforms = [
+                                             cellmagic(end_on_blank_line=line_input_checker),
+                                             leading_indent(),
                                              classic_prompt(),
                                              ipy_prompt(),
                                              strip_encoding_cookie(),
@@ -496,7 +498,7 b' class IPythonInputSplitter(InputSplitter):'
         if logical_line_transforms is not None:
             self.logical_line_transforms = logical_line_transforms
         else:
-            self.logical_line_transforms = [cellmagic(end_on_blank_line=line_input_checker),
+            self.logical_line_transforms = [
                                             help_end(),
                                             escaped_commands(),
                                             assign_from_magic(),