[MRG] Refactor and clean IsotonicRegression

Refactor and clean IsotonicRegression:

* Rename ``X`` to ``x`` to properly reflect that input is a vector, not matrix
* Refactor the ``y=isotonic_regression(...)`` logic into ``_build_y``
* Refactor the ``interp1d`` logic into ``_build_f``
* Store ``f=interp1d(...)`` so that it is not re-calculated on each call to ``transform``

Sorry about the last one, @agramfort.  Reset and sync'd.
@ogrisel, isn't returning ``order_inv`` a slightly more efficient way to execute ``fit_transform`` than calling ``transform`` from scratch?

See L364