Show serialization with pickle

Two things here:

* When pickle checks if the __getstate__ method exists, it expects to
have an AttributeError if it does not, so TVDBAttributeError should
inherit from AttributeError.

* When loading a serialized object, pickle will call __setstate__ before
restoring the data. As the 'data' attribute is not set at this time, a
infinite recursive call on __getattr__ will occur.