Dataframe creation: Specifying dtypes with a dictionary
Apologies if this feature has been suggested before. Many of the IO functions (e.g. `read_csv`) allow use to easily specify the format for each column using a dictionary. As far as I understand, this is **not**
 possible with the regular dataframe construction, e.g:

``` python
df = pd.DataFrame(data=data, columns=columns, dtypes={'colname1': str, 'colname2': np.int})
```

**Even better**, it would be great if one could change the `dtypes` for the dataframe columns using a similar contruction, e.g.:

``` python
df.change_types({'colname1': str, 'colname2': np.int})
```

Is anything like this planned for already? 
