ENH: create Timedelta scalar / TimedeltaIndex

closes #3009
closes #8187
closes #8209
closes #4533 
closes #8190
 
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``
@jorisvandenbossche no problem. will change it back. right now ``__dir__`` ignores only leading underscores (e.g. private). That's sort of the problem, we have: 

- private to that class hierarchy (e.g. only used internally in say Index/DatetimeIndex), clearly have to be ``_``
- public: meant to call externally
- semi-private: the public interface to Index called by Series/DataFrame. This is really what this is. It IS separate, IOW, you don't really want Series/DataFrame to reach into the internals of the Index (and DataFrame to Series and such). unless really necessary. Right now these are either public or private, no way to make them 'semi-private' (e.g. pandas internal only, but not internal to that hierarchy).

any thoughts?