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. 
Except, you want to validate, because if you request something like, say, UK, instead of GB (No idea what actually happens, or which one is right), how does one check that they got "the right" country?  World bank does tricky things. For instance, request a stale country code, and they'll they send a response back with the updated new one.  A user could manually look at the response from World Bank, but automating that check is very tricky, if you don't start with a known list of country codes.

The static, hard coded, map, works/worked really really well.  Exceptionally, explicitly, well.

For the sake of a few hundred, really, really, easy to understand lines of code (the map I already deleted), you're pushing to make pandas, and any user's code, less maintainable, as well as the response from World Bank, and the notifications Pandas provides with it, less explicit than I managed to create.

I'm a rookie at git, and the etiquette on github, but not logic. I've reviewed the problem of adding 3 digit country codes, very thoroughly.  Both reading World Bank, looking at their API, and referencing Wikipedia.