ENH: Store comments from read_csv in an property (`df.comment`) and allow saving
#### Problem description
This is an enhancement request to allow handling and saving comment strings with DataFrame text file IO. Two related stackoverflow questions about such feature: 
http://stackoverflow.com/questions/39724298/pandas-extract-comment-lines
http://stackoverflow.com/questions/29233496/write-comments-in-csv-file-with-pandas

#### Code Sample, a copy-pastable example if possible
```python
df = pd.read_csv("mydata.csv", comment="#")
df.comment
"this is a comment from mydata.csv file"
df.to_csv("output.csv", comment="#") # saves the `comment` string by pasting "#" before its each line and putting it before the table. 
```

#### Output of ``pd.show_versions()``
<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.0
nose: 1.3.7
pip: 9.0.1
setuptools: 29.0.1
Cython: 0.24
numpy: 1.12.0
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.0.0
sphinx: 1.4.5
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.6.0
matplotlib: 1.5.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.4.1
html5lib: None
httplib2: 0.9.2
apiclient: 1.5.1
sqlalchemy: 1.0.14
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: None
</details>
