Subclass support for handlers + decorator syntax for register()

This PR adds two features, both being improvements to the `jsonpickle.handlers.register` function:

1. An optional keyword argument `base`, disabled by default. If set to `True`, the handler will be invoked for all subclasses of the given class. If a class has a personal handler that was set explicitly, it will always take precedence (see tests).

2. Optional decorator syntax:
```python
@jsonpickle.handlers.register(Foo, base=True)
class FooHandler(jsonpickle.handlers.BaseHandler):
    pass
```

// there are also some minor pep8 fixes thoughout.