API: add DataFrame.nunique() and DataFrameGroupBy.nunique()
When exploring a data set, I often need to `df.apply(pd.Series.nunique)` or `df.apply(lambda x: x.nunique())`. How about adding this as `nunique()`-method parallel to `DataFrame.count()` (`count` and `unique` are also the two most basic infos displayed by `DataFrame.describe()`)?

I think there are also use cases for this as a `groupby`-method, for example when checking a candidate primary key for different lines (values):

``` python
>>> import pandas as pd
>>> df = pd.DataFrame({'id': ['spam', 'eggs', 'eggs', 'spam'], 'value': [1, 5, 5, 2]})
>>> df.groupby('id').filter(lambda g: (g.apply(pd.Series.nunique) > 1).any())
     id  value
0  spam      1
3  spam      2
```
