Refactor of prompts and the displayhook.

* Renamed CachedOutput to displayhook.DisplayHook.
* Added methods that DisplayHook.__call__ uses to do its work.
  These methods can now be overridden for the ZMQ kernel.
* Removed all hooks (result_display and generate_output_prompt)
  that the outputcache was using. We need to add these back in
  once we figure out what to do about hooks in general.

IPython/testing/iptest.py

@@ -250,7 +250,7 @@ class IPTester(object):
             return os.system(' '.join(self.call_args))
     else:
         def _run_cmd(self):
-            #print >> sys.stderr, '*** CMD:', ' '.join(self.call_args) # dbg
+            # print >> sys.stderr, '*** CMD:', ' '.join(self.call_args) # dbg
             subp = subprocess.Popen(self.call_args)
             self.pids.append(subp.pid)
             # If this fails, the pid will be left in self.pids and cleaned up