Issues with numeric_only for DataFrame.std()
The docstring shows a `numeric_only` option for `DataFrame.std()` but it does not seem to actually be implemented. I'm happy to take a crack at fixing it but I'm not sure whether it's the doc or the implementation that needs fixing.

To see this consider a mixed-type `DataFrame` where I'm setting one entry to be a `str` of `'100'` while all other entries are `float`. For `std()` It does not matter whether `numeric_only` is `True` or `False`, but for `max()` it clearly makes a difference. 

```
In [1]: import pandas as pd
In [2]: import numpy as np
In [3]: df = pd.DataFrame(np.random.randn(5, 2), columns=['foo', 'bar'])
In [4]: df.ix[0, 'foo'] = '100'

In [5]: df
Out[5]:
         foo       bar
0        100 -1.958036
1   0.221049  0.309971
2   1.200093 -0.103244
3  -2.475388 -2.279483
4  0.1623936 -1.185682

In [6]: df.std(numeric_only=True)
Out[6]:
foo    44.841828
bar     1.129182
dtype: float64

In [7]: df.std(numeric_only=False)
Out[7]:
foo    44.841828
bar     1.129182
dtype: float64

In [8]: df.max(numeric_only=False)
Out[8]:
foo    100.000000
bar      0.309971
dtype: float64

In [9]: df.max(numeric_only=True)
Out[9]:
bar    0.309971
dtype: float64
```
