pd.DataFrame.__deepcopy__ is broken
Looks like the pandas deepcopy is no longer fully recursive.  For example if we have nested lists as shown below, the most inner lists are actually exactly the same.  

```python
>>> x = [[1, 2, 3, 4]]
>>> y = [[2, 3, 4, 5]]
>>> import pandas as pd
>>> df = pd.DataFrame({'x':x, 'y':y})
>>> import copy
>>> df2 = copy.deepcopy(df)
>>> id(df2['x'][0])
4572099912
>>> id(df['x'][0])
4572099912
```

We first noticed this issue on [skbio](https://github.com/biocore/scikit-bio/pull/1530) when the copy unittests were failing.  This hasn't been a problem with the previous pandas release.  Looking at the commits in the previous release, this [one](https://github.com/pandas-dev/pandas/commit/0b4fdf988e3125f7c55aaf6e08a2dfa7d9e2e8a0) looks suspicious.

CC @gregcaporaso @ebolyen @jairideout 

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.3
pytest: None
pip: 9.0.1
setuptools: 27.2.0
Cython: None
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
