REPR: kwds in DateOffset
Currently the signature for constructing most `DateOffset` is `(n, **kwds)`.

Optional ags are then read using `get(key)` from the `kwds` and required args using `[key]`. This is fine, but it leaves what was extracted from `kwds`  still in `kwds` leading to a redundant `str` or `repr`: 

```
Week(weekday=0)
<1 Week: kwds={'weekday': 0}, weekday=0>
```

I would recommend using `pop` instead of `get` and using `del` after reading required values.

Thoughts?
