API/BUG: awkward syntax to add categories to a Categorical
from [SO](http://stackoverflow.com/questions/29709918/pandas-and-category-replacement/29712287#29712287)
- the `.add_categories` should be able to take an `Index`/ndarray, ATM it must be converted to a list
- should be a keyword for something like: add any additional categories that I am passing to you (even if the current ones are duplicates, just add these to the end). e.g.

```
In [147]: s = pd.Categorical(list('aabbcd'))

In [148]: s2 = list('aabbcdefg')

In [149]: s
Out[149]: 
[a, a, b, b, c, d]
Categories (4, object): [a, b, c, d]

In [150]: s.add_categories(s.categories.sym_diff(Index(s2)).tolist())
Out[150]: 
[a, a, b, b, c, d]
Categories (7, object): [a, b, c, d, e, f, g]
```

I would ideally just like to say:

```
s.add_categories(s2, take_new=True)
```

(maybe not the best keyword, but something like this)
