Code cleanup (unused imports, unused variables...)

[femtotrader]

Hello,

according to
https://landscape.io/github/pydata/pandas-datareader/
there is 2 "errors" (same line 75 in wb.py)

def download(country=['MX', 'CA', 'US'], indicator=['NY.GDP.MKTP.CD', 'NY.GNS.ICTR.ZS'], ...)

these 2 errors are

Dangerous default value [] as argument

There is also some "smells" and some "Style" issues.

Badge

can be inserted with the following reStructuredText code

.. image:: https://landscape.io/github/pydata/pandas-datareader/master/landscape.svg?style=flat
   :target: https://landscape.io/github/pydata/pandas-datareader/master
   :alt: Code Health

Kind regards

[femtotrader]

Dangerous default value [] error 

is fixed by http://github.com/pydata/pandas-datareader/pull/121

but some cleanup is still possible

[femtotrader]

It may be a good idea to add flake8 to .travis.yml

[femtotrader]

PR #162 being merged we can probably close this issue now.