BUG: datetime64 series reduces to nan when empty instead of nat
I ran into some strange behavior with a series of dtype datetime64[ns] where I called max and got back a `nan`. I think the correct behavior here is to return `nat`. I looked through test_nanops but I am not sure where the correct place to put the test for this is.

The new behavior is:

``` python
In [1]: pd.Series(dtype='datetime64[ns]').max()
Out[1]: NaT
```

where the old behavior was:

``` python
In [1]: pd.Series(dtype='datetime64[ns]').max()
Out[1]: nan
```

can I add a test in test_nanops and/or test_series
 also want to test for timedelta64/datetime64[ns, tz] too
 Sure, I will update this tonight when I get home
 pls also add a note in the 0.17.1 whatsnew bug section
 ping @jreback 
 question for you. otherwise lgtm, pls squash to a single commit (I know different from other projects, just convention here)
 Also I removed the workaround branch so hopefully the tests pass, otherwise we can put that branch back.
 ok looks good. I'll be merging things on friday after releasing 0.17.0
 Thank you very much
 @llllllllll no thank you!
 @jreback just rebased against master, good to merge?
 we have lots of prs that are worked on at the same time
so when someone adds a note in whatsnew if they are all at the end
merging one requires everyone to rebase
but if the notes are inserted in a big list they are resolved by git
so we can merge many things w/o conflicts 
 Ah, that's a cool trick
 minor comment, ping when green
 tests are passing
 thank you sir!
