Empty DataFrame constructor disregards dtype kwarg
This is new in 0.16.1

``` python
In [1]: import pandas as pd

In [2]: pd.__version__ 
Out[2]: '0.16.1'

In [3]: f = pd.DataFrame({'a': [0, 10, 20, 30, 40], 'b': [5, 4 ,3, 2, 1]},
                          index=[1, 2, 3, 4, 4])

In [4]: f.dtypes 
Out[4]: 
a    int64
b    int64
dtype: object

In [6]: pd.DataFrame(columns=['a', 'b'], dtype=f.dtypes).dtypes
Out[6]: 
a    object
b    object
dtype: object
```

Also other mechanisms to convey dtype information yield uninformative errors

``` python
In [7]: pd.DataFrame(columns=['a', 'b'], dtype={'a': 'i4', 'b': 'f4'}).dtypes
ValueError: entry not a 2- or 3- tuple

In [8]: pd.DataFrame(columns=['a', 'b'], dtype=[('a', 'i4'), ('b', 'f4')]).dtypes
NotImplementedError: compound dtypes are not implementedin the DataFrame constructor
```

Is there a way to create an empty DataFrame with given dtypes in 0.16.1?
