FIX value_counts should skip NaT
fixes #7423  
fixes #5569.

Hmm this seems to be explicitly tested for in `test_base` and `test_timeseries`. I still think correct behaviour is to drop the NaTs...
 The test was added based on the discussion in #6734. I agree it will be clearer if `NaT` is treated as the same as `nan`. 
 so this makes this consistent with `nan` (which is excluded), separately, maybe have an option to `value_counts` to _include_ `nan/NaT`?
 @jreback yes, shouldn't be too trick to put something together. I was thinking the same when I was doing this... of some analogous `include_na` (maybe `get_dummies`... called `dummy_na`)... I suppose `dropna` is more standard kwarg.
 yes, `dropna=True` (as the default) is prob correct. Pls add this. I think we might want to change the default to `dropna=False` in 0.15.0 but that is a separate issue.
 @hayd looks good...squash and merge
 thanks!
 @hayd some comments, but will fix it in a PR
 @jorisvandenbossche thanks! good points!
