Add DST handling

Hi, you'll find in this pull request a small change to Authenticator.
I live in France, where we use Daylight Saving Time: the local time is GMT+1 half of the year, and GMT+2 the other half.
Since it is not easy to deduce from local time if we are in DST or not (and even not possible 1 hour each year), I just made a little change to let the user togle from/to DST.

This will add or retrieve 1 hour to the computation of GMT from localtime each time the select button is pressed.

I have added a label displaying which GMT offset is used to clarify things.