None not always considered missing
I expected pandas to know that `None` should be ignored as a missing value in the following simple example:

```
import pandas
import numpy

# works fine
df = pandas.DataFrame({"col1":[2,5.0,123,numpy.nan], "col2":[1,2,3,4]})
print df.sub(3)

# fails -- tries to do the subtraction with None
df = pandas.DataFrame({"col1":[2,5.0,123,None], "col2":[1,2,3,4]})
print df.sub(3)

```
