BUG: min on non-numeric data with nans
related #4279
related #5967

Min doesn't seem to work as expected with NaNs and non-numeric data:

```
In [1]: s = pd.Series(['alpha', np.nan, 'charlie', 'delta'])

In [2]: s
Out[2]:
0      alpha
1        NaN
2    charlie
3      delta
dtype: object

In [3]: s.min()  # by default docstring suggests this should skipnan
Out[3]: inf
```

The hack/workaround is to exclude them, perhaps we should special case this in the code:

```
In [4]: s[s.notnull()].min()
Out[4]: 'alpha'
```

_From discussion on [this pandas SO question](http://stackoverflow.com/a/17497460/1240268). and I also posted a corresponding numpy issue:
https://github.com/numpy/numpy/issues/3508_.
