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. 
I think I understand your logic.  But because I think I do, and cause of my note below, I did 37f90b7 instead.

It will fail consistently, it's just a question of which exception catches it.  Since the messages to both contain the same string, and are both ValueErrors, this test should always "work".

The only thing that would cause the test to stop working would be if WB unretired the indicator or their API changed. 

More Info:

The indicator is a retired one. If World Bank removed it, from their indicator list, it would get caught at line 161 (all indicators failed), rather than where it gets caught now at 201 (single indicator query failure).