Inherit fife.Exception from python's (Base)Exception

As outlined in [PEP-352], exceptions should be properly derived from a
base exception class. With `%exceptionclass` SWIG will generate this:
```py
    class Exception(Exception):
```
in fife.py where before it generated this:
```py
    class Exception:
```
The SWIG default is to support python versions earlier than 2.5 (where
BaseException was introduced), so it uses python.Exception here.
With Python 2.5, Exception was made a subclass of the newly introduced
BaseException, so this still does the right thing and I'm not concerned.

[PEP-352]: http://www.python.org/dev/peps/pep-0352/