BUG: Panel does not preserve DataFrame dtypes
#### A small, complete example of the issue

``` python
>>> df = pd.DataFrame({'a':[1,2,3], 'b':[True,False,True]})
>>> df.dtypes
a    int64
b     bool
dtype: object

>>> pl = pd.Panel({'x':df})
>>> pl.x.dtypes
a    object
b    object
dtype: object
```
#### Expected Output

I expected the dtypes of `df` and `pl.x` to match, the latter was created directly from the former.
#### Output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-85-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
