ENH: create Timedelta scalar / TimedeltaIndex

closes #3009
closes #8187
closes #8209
closes #4533 

This PR creates a new ``timedelta`` sub-class, the scalar ``Timedelta``, very similar to how ``Timestamp`` sub-classes ``datetime``. Further it creates a ``TimedeltaIndex`` modelled on ``DatetimeIndex`` that holds the underlying data (boxing is only on iteration).

- ToDo
  - Document API changes:
     - [x] ``to_timedelta`` return ``TimedeltaIndex`` rather than a ``Series``
     - [x] ``to_timedelta`` with scalar returns ``Timedelta`` (rather than ``np.timedelta64``)
     - [x] Boxing for ``list(Series(...))`` is now ``Timedelta``
     - [x] possible repr changes
     - [x] ``timedelta_range``
     - [x] usage / examples
     - [x] API
  - Timedelta
      - [x] conversions ``to_timedelta/to_pytimedelta``
  - TimedeltaIndex
      - [x] conversions ``to_pytimedelta``
  - [x] what else needs boxing in Series?
  - [x] fix formatters (Timedelta64Formatter; remove ``repr_timedelta64`` (no longer necessary)
  - [x] create ``timedelta_range`` & fix freq / inferred_freq
  - [x] fix remaining copied methods from DatetimeIndex
  - more tests (of TimedeltaIndex / scalar)
      - [x] scalar comparisons
      - [x] index comparisons
      - [x] getitem / partial string access
      - [x] sql expecting ``np.timedelta64`` ? (currently skipping these 2 tests!!!!), see: https://github.com/jreback/pandas/commit/2743ef16f7dfe55ba63663fa65c5671031af94f1
      - [x] validate series / td ops
  - move methods of ``TimedeltaIndex/DatetimeIndex``
      - [x] move ``DatetimeOpsMixin`` to ``tseries/base.py``?
      - [x] ``shift/take/order``
      - [x] ``slice_indexer``
      - [x] ``append/union/intersection``
      - [x] comparbless?
      - [x] ``.dt`` accessor are ok now
      - [x] move ``.td`` to ``.tdi``
I didn't want to change the printing format. (plus always printing 9 digits kind of a lot). And the problem is you can't selectivitly do it (except at the very top level where we have some of this logic). Maybe in the future can address these extra wide printing