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. 
As above, see 37f90b7.  Is that better? I think it is.  If it isn't, I'll put SkipTest in tonight - I gotta fly to my 9-5.

At 10,000 feet, these tests are designed to catch when/if WB change how they return errors.  If they do, the error logic to pandas users might need changing.