Add keyword-aliasing for underscores

This fixes #6 by stripping underscores from attributes for the initializer keyword-arguments.

It does it only for the new-style `Attribute` classes because I have the feeling that adding it retroactively to string-based ones would break backward-compatibility big time (I’ll happily listen to contrary opinions :))

What this does is basically the following:

```python
@attributes([Attribute("_private")])
class C(object):
    pass

c = C(private=42)
assert c._private == 42
```

I would appreciate both feedback whether this is what you wanted *and* for a better name for `keep_underscores` because the name is gross. :|