Flag switch case statements that are not explicit in what they do

I got burned by a case statement that didn't fallthrough.  This will flag case statements that don't have an explicit action associated with them and/or appear to not do any actual work.  testdata/switch.go is pretty straightforward.

I ran this across a wide range of packages and didn't find any obvious failures, although a good number of false positives appear.  They generally appear to be an intentional use of the lack of implicit fallthrough (at least when commented).  Others are suspect.  Personally, I write a break statement in when I mean it.

(Note: I did not commit golint.go w/ a new package reference if you're testing this)