added strideable methods

[michaeleisel]

this allows stuff like

for date in stride(from: date, to: graphEndDate, by: 1.hour) {
    addHourLabel(forDate: date)
}

let range: StrideTo<NSDate> = stride(from: dateRange.startDate, to: dateRange.endDate, by: 10.minutes)
return map(range, { date in
    //...
})