CLN: replace pandas.compat.scipy.scoreatpercentile with numpy.percentile
PR to fix #5824.
- Replaces `compat.scipy.scoreatpercentile` with `numpy.percentile`.
- Sets `axis=0` in a few tests, because `axis=None` by default on `numpy.percentile`, which returns a scalar (i.e., operates on a flattened version of the array).
- Fixes a test that fails after the switch due to differences in how fractions are computed. (Created a `np.timedelta64` directly since `to_timedelta` doesn't support enough precision.)

Let me know if you'd like to see any changes..

pls add a release note referinc the issue (do it in API changes, even though just a compat change).
 side note, I am trying to figure out why @wesm included this in the first place, as I think `np.percentile` has been around a while? any idea
 @jreback, sorry for the delay, but I've added a second commit to this PR with:
- added to release notes
- `Series.quantile` on datetime[ns] series changed to return a `Timestamp` object (with an explicit test)
- added a test for `Series.quantile` on timedelta64 series, which didn't require any
  special handling, though the test is skipped at numpy 1.6.x, because
  numpy.percentile throws the following exception with timedelta64 objects
  pre-1.7:
  
  ```
      TypeError: ufunc 'multiply' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule 'safe'
  ```
- removed timedelta64 handling (introduced by #6558) in `Series.quantile`,
  because `not np.isscalar` is always false and, in my testing,
  numpy.percentile properly returns timedelta64 objects on numpy 1.7 & up.

No idea why np.percentile wasn't used originally, but will note that scipy.stats.scoreatpercentile existed first. Perhaps, when the scipy dependency was removed from pandas, the difference in rounding made copying scoreatpercentile the easiest way forward.  Will also note that scipy has deprecated scoreatpercentile in favor of np.percentile.
 @gdraps looks good

just a couple of minor comments

timedeltas are thoroughly broken under numpy < 1.7
so I gave up on trying to make them work a while ago
 @jreback, updated the last commit of this PR based on your feedback -- let me know if you see anything else
 @gdraps thanks for the work on this gr8!
