WPI: create Timedelta scalar / TimedeltaIndex

closes #3009

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:
     - [ ] ``to_timedelta` return ``TimedeltaIndex`` rather than a ``Series``
     - [ ] Boxing for ``list(Series(...))`` is now ``Timedelta``
     - [ ] usage / examples
  - Timedelta
      - [ ] conversions ``to_timedelta``,``to_timedelta64``
  - [ ] what else needs boxing in Series?
  - [ ] fix formatters (Timedelta64Formatter; remove ``repr_timedelta64`` (no longer necessary)
  - [ ] create ``timedelta_range`` & fix freq / inferred_freq
  - [ ] fix remaining copied methods from DatetimeIndex
  - more tests (of TimedeltaIndex / scalar)
      - [ ] scalar comparisons
      - [ ] index comparisons
      - [ ] getitem / partial string access
      - [ ] sql expecting ``np.timedelta64`` ?