Minor cleanups for nanops

[toddrjen]

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

[toddrjen]

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.

[jreback]

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_...)

[toddrjen]

This seems to be ready now. travis is green and the first patch has been squashed.

[jreback]

can run a perf check and post anything big
(should be ok but just checking)

[toddrjen]

There are no major performance regressions.

[jreback]

thanks!