GOTCHA: You can't use dot notation to add a column to a DataFrame
I ran into a nasty gotcha I have not seen mentioned in the documentation.  In general, you can refer to DataFrame columns like a dictionary key or like an object attribute.  But the behavior is inconsistent.  In particular, you can modify an existing column using dot notation, but you can't add a new column.

The following example demonstrates.

```
d = {'one' : pd.Series([1., 2., 3.], index=['a', 'b', 'c']),
     'two' : pd.Series([1., 2., 3., 4.], index=['a', 'b', 'c', 'd'])}
df = pd.DataFrame(d)

# this adds a column to df
df['three'] = df.two + 1

# this successfully modifies an existing column
df.one += 1

# this seems to work but does not add a column to df
df.four = df.two + 2

print df.three
print df.four
print df
```

Is this behavior unavoidable?
