mean over axis=1 in not-completely-numeric frame returns all nans
DataFrame from #3688. This might be related to that:

``` python
df = pd.DataFrame({'bar': {0: 1, 1: 1, 2: 1}, 'foo': {0: 0, 1: 1, 2: 2}, 'foo1': {0: 1, 1: 2, 2: 3}, 'hello': {0: 'a', 1: 'a', 2: 'a'}}, columns=['bar', 'foo', 'foo', 'hello'])
print df.T.sum(1) == df.sum()  # fine. + is str cat
print df.T.mean(1).isnull().all()  # prints True
```

I think non numeric should be dropped...
