Add datetime2unix(::ZonedDateTime) method

[CameronBieganek]

Expected behavior:

julia> using Dates, TimeZones

julia> t = ZonedDateTime(2014, 7, 15, 14, 35, 46, tz"America/Chicago")
2014-07-15T14:35:46-05:00

julia> datetime2unix(zdt::ZonedDateTime) = datetime2unix(DateTime(astimezone(zdt, tz"UTC")))
datetime2unix (generic function with 2 methods)

julia> datetime2unix(t)
1.405452946e9

[giordano]

The functionality is already there without defining custom functions:

julia> using TimeZones

julia> t = ZonedDateTime(2014, 7, 15, 14, 35, 46, tz"America/Chicago")
2014-07-15T14:35:46-05:00

julia> TimeZones.zdt2unix(t)
1.405452946e9

Note that PR #224 implements the interface that you'd like to have