CLN/INT: remove Index as a sub-class of NDArray

make ``Index`` now subclass ``PandasObject/IndexOpsMixin`` rather than ``ndarray``
should allow much easier new Index classes (e.g. #7640)

This doesn't change the public API at all, and provides compat (with exception of a very minor ``DatetimeIndex`` comparison with ndarrays)

closes #5080, preserves pickle compat for >= 0.8.0
back compat for pickles is now way simpler

This removed pickle compat for < 0.8.0 (well I think it does but can't really prove it). maybe can add it back if strong interest.

ToDo:

- docs
  - [x] release note
  - [x] warnings in io.rst about this change
  - [x] pickle compat now > 0.8.0
  - [x] fixes ``.repeat`` on MultiIndex (broken in master)
  - [x] minor API compat issue with comparisons of ``DatetimeIndex`` with ``NaT`` vs ndarrays
- [x] merge with searchsorted issues/PR ( #6712, #7447, #6469)
- [ ] tests fixed (FIXMES), just a few left
- [x] perf
- [ ] json completely broken ATM
- [x] #7796 FIXME (PeriodIndex not supported in HDF), not really a big deal
- [x] #7439 since ``Index`` now doesn't have implicit ops (aside from ``__sub__/__add__``, these need to be added in (e.g. ``__mul__,__div__,__truediv__``).
- [x] bool(Index/MultiIndex) : https://github.com/pydata/pandas/issues/7897 (will address this later)
@shoyer I have determined that this CAN be done by intercepting (and interpreting the ``context``) in the ``__array_preprare__`` call (similar to what is done in ``core/series.py/__array_prepare__``). However IMHO this is pretty complicated (as you would need to translate the ufunc and reverse the arguments). Leaving it off for now with just the docs warning. I think this is a very limited case anyhow.