nonexistent=shift in tz_localize not precise
#### Problem description

As of today (2018-12-28), I read on http://pandas-docs.github.io/pandas-docs-travis/timeseries.html#nonexistent-times-when-localizing that 

> A DST transition may also shift the local time ahead by 1 hour creating nonexistent local times. The behavior of localizing a timeseries with nonexistent times can be controlled by the nonexistent argument. The following options are available:
> 
> raise: Raises a pytz.NonExistentTimeError (the default behavior)
> NaT: Replaces nonexistent times with NaT
> shift: Shifts nonexistent times forward to the closest real time

This is a great new feature (i.e. having leeway to manage NonExistentTimeError explicitly)!
To be sure I understand the problem of NonExistentTimeError correctly, is it correct to state they appear if an only if the time happens during a DST change (jumping one hour ahead) ? e.g. if I take tz=CET, we had the DST on the 2018-03-25, with the hour [02:00->03:00[ not existing so any localization of such time will raise a NonExistentTimeError:
```python
Timestamp("2018-03-25T02:33:00").tz_localize("CET")`)
# pytz.exceptions.NonExistentTimeError: 2018-03-25 02:33:00
```
Or are there other cases ?

If so, I see the following behavior that would be useful besides the 'shift':
- 'shift_backward' / 'shift_forward' ==> Shift nonexistent times backward/forward by one hour. Example of use case for shift_backward: I do some calculation on a local timestamp without tz, that I shift by 2 hours backward (to say "take it two hours before") and then I localize and get a NonExistentTimeError (e.g. `Timestamp("2018-03-25T04:33:00") - DateOffset(hours=2)`). I would like to get as a result of the tz_localize('CET'), the time "2018-03-25T01:33:00+0100" or "2018-03-25T03:33:00+0200" (and not "2018-03-25T01:59:59.99999+0100" or "2018-03-25T03:00:00+0200" as with 'snap_*')
- 'snap_backward' / 'snap_forward' => Shifts nonexistent times backward/forward to the closest real time (ie explicit the direction of the shift). Example of use case for snap_backward: if I want to get the value of something known at some instant T and T is not existent, I would rather prefer to have the value at some instant T* < T (='backward') to avoid having "forward looking information". However, the 'snap_backward' is ill defined for the DST as the closest backward time for 2018-03-25T02:33:00 in CET is 2018-03-25 01:59:59.999999999...=2018-03-25 02:00:00 which is non existent... I guess this is the reason why the current 'shift' only propose the forward version, correct?

I miss essentially the 'shift_backward' ability in my day to day cases.