ENH: Add ISO3 Codes & safe_cc_only to io.wb.download #8482

This PR adds ISO 3-digit country codes, as well as an argument safe_cc_only to io.wb.download.

closes #8482 .

I will squash all, when appropriate/finished.  Looking forward to feedback.

#### Testing?

I'm a beginner when it comes to nose, testing and virtual environments. I've never used tox.
I manually tested wb.download(), from wb.py in Python 3.4 and Python 2.7.  I'm pretty positive that its unlikely that tests have been set up for wb.download anyway, but, I did run test.sh on Python 3.4.  

#### Version?

Please tag the issue, or this PR, with a version (0.15.1? 0.16?), so that I can update the right parts of the documentation. 
The map, is basically just there for reference.  Improve maintainability.

The issue is that World Bank says one thing, then does another.  They say "ISO", but then add/adapt that, as they see fit sporadically.  Then, even the ISO standards, don't do a great job of creating a static standard. Because countries squabble and struggle to find a common ground, they create exceptions, borders change, it's dynamic.  

This way, we hardcode the "latest" World Bank Standard, but we give the option to the users to work around World Bank's flakey logic if changes.  

See "KSV", for example.  Not in the ISO standard, but its the only way to get data for Kosovo.  The user would set safe_cc_only to False, and be able to get data, without the available warnings.

Also, users could very easily adjust the list, and easily look up country codes in wb.io.country_codes.