options on child views and automatically accumulating _keys from superclasses

I added two more things I needed in my backbone-flat branch:

1. options when creating child views (I needed this for my Controls view that let you spin a 3d model around)
2. explicit setting of the keys attribute, which makes it very simple and obvious what keys will be synced to the server.

(actually, one commit makes keys automatically accumulate _keys from superclasses, which fixes some problems with the previous solution when you have big inheritance hierarchies.  However, I think it's too much magic, and that we should be more explicit about exactly what keys get synced to the server.)

In Python 3, we could have a nice magic solution for automatically accumulating base class's keys by doing something like:

```python
@property
def keys(self):
    return self._keys + super()._keys
```
However, this doesn't work in Python 2 very well, since you have to specify the current class in the `super()` call.