Accessing all user-created metadata of a pandas Series
#### Code Sample, a copy-pastable example if possible

```python

import pandas as pd
import numpy as np

s = pd.Series(np.random.randn(10))
s.foo = 'bar'

print(s.__dict__)

```
#### Problem description

I heard on [SO](https://stackoverflow.com/questions/14688306/adding-meta-information-metadata-to-pandas-dataframe) that there was some sort of `metadata` field. I was not able to figure out how to work with this.

On the other hand, I *was* able to access metadata through the `__dict__` attribute. However, I got lots of other properties as well. I could write something like

```python
def get_metadata(series):

    tmp_dict = dict()
    for key in s.__dict__:
        if key.startswith('_'):
            continue
        if key == "is_copy":
            continue
        tmp_dict[key] = s.__dict__[key]
    
    return tmp_dict

get_metadata(s)
```
But it seems like this is something that could ship with pandas out of the box.

#### Expected Output

This is what I'm currently getting

```
{'foo': 'bar', '_item_cache': {}, '_name': None, '_data': SingleBlockManager
Items: RangeIndex(start=0, stop=10, step=1)
FloatBlock: 10 dtype: float64, 'is_copy': None, '_index': RangeIndex(start=0, stop=10, step=1), '_subtyp': 'series'}
```

This is what I'd like to see, perhaps through another function. (`metadata`?)

```
{'foo': 'bar'}
```

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.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: 0.19.1
nose: None
pip: 9.0.1
setuptools: 36.2.7
Cython: None
numpy: 1.11.2
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b10
httplib2: 0.10.3
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
boto: None
pandas_datareader: None


</details>
