Datetime resolution coercion
Fixes #10245 

Without this fix, if the `date_parse` function passed to e.g. `read_csv` outputs a `np.datetime64` array, then it must be of `ns` resolution. With this fix, all time resolutions may be used.

For some reason 706b8df fails Travis on my fork. I can't figure out why. The error doesn't seem to be related to my testing function (which was the only thing I changed, apart from importing `MultiIndex` at the top).
 pull from master again and rebase. i fixed the issue.
 Still failed Travis in my fork
 oh, that was a generic message I sent out. You might have a specific issue then.
 Then I really don't understand what's going on.
- 1549094 [passes](https://travis-ci.org/cmeeren/pandas/builds/65068780) (after fixing the issue and making the test)
- 706b8df [fails 1/5](https://travis-ci.org/cmeeren/pandas/builds/65374905) (after I updated my test) fails in a test I haven't touched 
- 3c348dd [fails 4/5](https://travis-ci.org/cmeeren/pandas/builds/65395818) (after rebasing) fails with another error

I'm at a loss here.
 try `git rebase -i origin/master`
and see https://git-scm.com/book/en/v2/Git-Branching-Rebasing
 There we are. Ended up adding pydata/pandas as a remote and rebasing this PR branch on top of that. Diff seems correct now.
 Build passed, will this be merged? Or is anything else needed?
 @cmeeren thanks for the nudge. I think this looks pretty much good now. All we need now is a release note in what's new in `0.16.2.txt`. You can put this under bug fixes.

Once you've done that, please squash to one commit.
 Oh no, again the thing with the wrong commits being included on rebase. I'll rebase onto `upstream/master` and try again.
 There we are.
 thanks!
 Oh, I thought you wanted the issue number in the "whatsnew" document. I hadn't pushed that yet. But you're the boss. :)
 I forgot I asked you, and just did it. https://github.com/pydata/pandas/commit/0a428e7cc9915717eaf0b99c365561c847ea4ef0

thanks!
 :+1:
