Fix error seen in combinedopenended modules

Had to change graceperiod to be a Timedelta, as it is in:

```
common/lib/xmodule/xmodule/capa_module.py:97:    graceperiod = Timedelta(
common/lib/xmodule/xmodule/peer_grading_module.py:51:    graceperiod = Timedelta(
lms/xmodule_namespace.py:40:    graceperiod = Timedelta(
```

Also, change `Timedelta`s to be immutable field types. This will alleviate a lot of the errors we see because the Timedelta class doesn't do json roundtrips correctly, and only mutable types are marked as dirty when we access them (immutable types are only marked as dirty on a __set__). Plus, Timedeltas ARE immutable, so this is safe. See http://docs.python.org/2/library/datetime.html#available-types

@cpennington 

Not sure who else to tag