Adding local_to_utc_dst function.

Currently, converting a time that occurs twice in a given region will only give one UTC/GMT time back, rather than two. That is, 1 AM November 3rd occurs twice in "America/New York", but the erlang_localtime library provides only one value back, GMT 6 (rather than GMT 5 and 6).

I'd like a way to get both. Erlang's calendar module provides this through a second function call, local_time_to_universal_time_dst (whereas the call that returns just one time omits the DST part), which seems a reasonable approach.

Pull request refactors out local_to_utc into also supporting a local_to_utc_dst function,