Minor cleanups for nanops
This includes the requested fix for `nanops`, using `common.is_*_dtype` functions whenever possible.

This can be easily split into two pull requests if you want.  This seems like a very small change so I wasn't sure.  

I can also squash the first two commits. I felt it was better at least for the review to have them separate for clarity, but before the actual merge I would guess you would probably want them squashed.
 this is ok
just squash and ping me on green

if u r interested ther are lots of dtype comparisons which need fixing to use these functions all over the library (eg dtype == object) is pretty prevalent 

also u can directly import these (instead of using com.is_...)
 This seems to be ready now.  travis is green and the first patch has been squashed.
 can run a perf check and post anything big
(should be ok but just checking)
 There are no major performance regressions.
 thanks!
