Subclass of DataFrame losing properties on arithmetic functions like __add__
```python
class PDMixIn(object):

    @property
    def _constructor(self):
        return self.__class__

class DF(PDMixIn, pd.DataFrame):
    _metadata = ['my_prop']

    def __init__(self, *args, **kwargs):
        my_prop = kwargs.pop('my_prop', 'some_prop')
        super().__init__(*args, *kwargs)
        self.my_prop = my_prop

df = DF(my_prop='new_prop')
print(df.my_prop)
# 'new_prop' 

# But after adding 1
df = df + 1
print(df.my_prop)
# 'some_prop'
```
Seems like DF is losing property value after addition I found similar behaviour for __sub__ and others.

I was able to solve the problem by adding following code to PDMixIn

```python
def __add__(self, other):
        result = pd.DataFrame.__add__(self, other)
        return self._constructor(result).__finalize__(self)

df = df + 1
print(df.my_prop)
# 'new_prop'
```
<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.22.0
pytest: 3.3.0
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.1
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.1.13
pymysql: None
psycopg2: 2.7.3.2 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
