Problem reading HTML tables with colspan
Hi, guys! First of all, thanks for making this open source! =D Keep up the good work!

**SCRIPT**

```
import pandas as pd

dfs = pd.read_html('https://en.wikipedia.org/wiki/List_of_states_and_territories_of_the_United_States')

print(dfs[0])
```

The output we found where  there is any colspan is considering only the first column, and the other values are pulled back in relation to the colspan amount, making the values on the tail's row as "NaN"

**#### Expected Output**
In this web page, there is a colspan in the "capital" and "largest city" columns when they are the same, I think that in a general way we would expect that  the value would be duplicate on the following <colspan's amount> columns in the generated DataFrame
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 15 Stepping 13, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 27.2.0
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.0
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: 0.2.1
