Observe dot separated keys

This allows to use the API mentioned in #257 (issue also reported in #296) for keys in an Observe that contain a `.`:

```js
var ob = new can.Observe();

ob.attr('a.b', 'test');
ob.attr() // -> { a: { b: 'test' } }

ob.attr({
    'a.b': 'test'
});

ob.attr() // -> { 'a.b': 'test' }

ob.attr('a.b')
// will look for 'a.b' first
// and then for ob.attr('a').attr('b')
```