Assignment by Attribute has Odd Behavior
When assigning to a column in a DataFrame, If I assign an existing column, the assignment works:

```
In [36]: df = pd.DataFrame({'a':[1,2,12], 'b':['a','b','a']})
In [37]: df.b = df.a
In [38]: print df
    a   b
0   1   1
1   2   2
2  12  12

[3 rows x 2 columns]
```

but assigning to a new column fails silently:

```
In [40]:df = pd.DataFrame({'a':[1,2,12], 'b':['a','b','a']})
In [41]:df.c = df.a
In [42]:print df
    a  b
0   1  a
1   2  b
2  12  a

[3 rows x 2 columns]
```
