Feature request: well-defined behaviour around DST thresholds

I would love to have clearly defined and predictable behaviour w.r.t. daylight saving time.

Example for CronTrigger in rough pseudo-code

```
from pytz import utc
resolution = timedelta(seconds=1)

...

def get_next_fire_time(self, previous_fire_time, now):
dt = max(self.start_date, previous_fire_time, now).astimezone(utc)
while dt <= self.end_date:
if self.is_valid_fire_time(dt):
return dt
date += resolution

def is_valid_fire_time(self, dt):
dt = dt.astimezone(self.timezone)
return dt is something that can be matched by the field expressions
```

As an example, consider a trigger to be executed at 2:30 CET. The above code states that if
- hour 2-3 is "missing" on March 27th 2016 due to a switch from winter to summer time, the next fire time after fire time 2:30 CET on March 26th will be be 2:30 CET on March 28th (leaving out the missing hour) and if
- the clock is set from 3 to back to 2 (hour 2-3 being repeated) on October 25th 2015 due to a switch from summer to winter time, we would have fire times 2015-10-24T02:30 summer time, then 2015-10-25T02:30 summer time, then 2015-10-25T02:30 winter time (the repeated hour), and then 2015-10-26T02:30 winter time.

This is not an urgent request, as I patched this in my fork of your apscheduler.

Before apscheduler, I used cron in UTC to execute each task in summer AND winter time and then exit the task if the CET time would not match what I expected (just like is_valid_fire_time in the pseudo code above).
I will never go back to cron. Apscheduler is so much more elegant. :-)