BUG: Add DataFrame column with dot notation fails
It seems like something weird is going on with the example below.

I often create a new column by mapping an old column. If I use dot notation with the creation of the new column the column is created and is accessible using dot notation, but it isn't listed as a column and can't be indexed the normal way (i.e. df['idx'])

Is this a bug or should I not be doing this?

Thanks, and btw pandas is terrific. This is my first bug report so apologies if its not helpful.

``` python
import pandas as pd

df=pd.DataFrame({'x':[1.,3.,5.],'y':[4,1.,7]})

df.columns
Out[3]: Index([x, y], dtype=object)

df.log_x=df.x.map(log)

df.columns
Out[5]: Index([x, y], dtype=object)

df.log_x
Out[6]: 
0    0.000000
1    1.098612
2    1.609438
Name: x

df['log_x']=df.x.map(log)

df.columns
Out[8]: Index([x, y, log_x], dtype=object)

df.log_x
Out[9]: 
0    0.000000
1    1.098612
2    1.609438
Name: x
```
