Attribute access to DataFrame behaves incorrectly
```
In [80]: dat = pd.DataFrame({'a' : [1,2,3], 'b' : [4,5,6]})

In [81]: dat
Out[81]:
   a  b
0  1  4
1  2  5
2  3  6

[3 rows x 2 columns]

In [82]: dat.c = 1

In [83]: dat
Out[83]:
   a  b
0  1  4
1  2  5
2  3  6

[3 rows x 2 columns]

In [84]: dat['c'] = 2

In [85]: dat
Out[85]:
   a  b  c
0  1  4  2
1  2  5  2
2  3  6  2

[3 rows x 3 columns]

In [86]: dat.c
Out[86]: 1

In [87]: pd.version.version
Out[87]: '0.13.0rc1-64-gceec8bf'
```
