Fix bug in serializing arguments of tasks that are more complex objects (#72)

* Give more informative error message when we do not know how to serialize a class.

* Check that passing arguments to remote functions and getting them does not change their values.

* fix serialization bug

* fix tests for common module

* Formatting.

* Bug fix in init_pickle_module signature.

* Use pickle with HIGHEST_PROTOCOL.

src/photon/photon_extension.c

@@ -125,6 +125,8 @@ PyMODINIT_FUNC initlibphoton(void) {
   m = Py_InitModule3("libphoton", photon_methods,
                      "A module for the local scheduler.");
 
+  init_pickle_module();
+
   Py_INCREF(&PyTaskType);
   PyModule_AddObject(m, "Task", (PyObject *) &PyTaskType);