decimal_date doesn't handle daylight savings time boundaries. #450

The decimal_date function doesn't seem to handle daylight savings time boundaries correctly.

> date4 <- ymd_hms("2016-03-13 01:00:00", tz = 'America/New_York')
> date4        <- date4 + dhours(-1:1)
> decimal4     <- decimal_date(date4)
> diff(decimal4)
[1] 0.0001138434 0.0002276867

The times above are a fixed hour apart however the decimal_date function ignores the tz information. Not sure this is intentional but it doesn't seem necessary unless I'm missing something.

I've fixed the issue in my fork and and wrote a couple tests. All tests in test-decimal-date passed. I'll issue a pull request shortly.