DISP: show column dtype in DataFrame.info() output
closes #3429.

Old:

```
In [39]: DataFrame(randn(10,2)).info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 10 entries, 0 to 9
Data columns (total 2 columns):
0    10  non-null values
1    10  non-null values
dtypes: float64(2)
```

New:

```
In [39]: DataFrame(randn(10,2)).info()
<class 'pandas.core.frame.DataFrame'>
Int64Index: 10 entries, 0 to 9
Data columns (total 2 columns):
0    10 non-null float64
1    10 non-null object
dtypes: float64(1), object(1)
```
