BUG: nanops.nanmedian doesn't work when axis=None
The default value for the `axis` argument for `nanops.nanmedia` is `None`.  However, the function raises an exception when using this value:

``` Python
>>> from pandas.core import nanops
>>> import bottleneck
>>> import numpy as np
>>> nanops._USE_BOTTLENECK = False
>>> 
>>> val = np.random.randn(13, 11, 7)
>>> 
>>> bottleneck.nanmedian(val, axis=None).ndim
0
>>> nanops.nanmedian(val, axis=None)
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'int'
>>> nanops.nanmedian(val)
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'int'
```
