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. 
> all tests should go with the other tests...

Why do some tests ```raise nose.SkipTest```? I'm not 100% positive, but I don't think the tests, as written before this PR, would have passed.  I'm guessing somebody dropped them into the tests, because the old tests were poorly written(?) 

I commented them out.  Please advise if I should uncomment, and what's the logic.