ENH: Interpolate NaT (GH-11701)
Implemented using the method described in the StackOverflow answer (http://stackoverflow.com/questions/33921795/fill-timestamp-nat-with-a-linear-interpolation/33922824#33922824).

It does lead to precision issues when converting `to_numeric` and back `to_datetime`. 

Wondering if there is a better way to do this?
