ENH: Enable passing in dict to specify datatypes to DataFrame constructor

The following seems like very natural syntax to specify the dtypes of various columns passed into the DataFrame constructor:

```
df = pd.DataFrame({'a':[1,2,3], 'b':[2.5,3.5,6.7]}, dtype={'a':'int32','b':'float32'})
```

However, right now it raises an error.  Perhaps something worth implementing?  See also http://stackoverflow.com/questions/21197774/assign-pandas-dataframe-column-dtypes 
