Add timezone support

[mannylopez]

Closes #21 and closes #28

Adds timezone support to TinyMoon and fixes a major flaw within the library.

Timezones are needed to calculate an exact time. Before, everything was calculated off of the current user's system timezone, leading to incorrect results.

I added extensive timezone tests: f4773e5#diff-86442e8ab0a16ee48438bbc6f15bed110625d172a752c18b258604cf840ff9d8

Now that I am using a Date with timezones, I can remove a lot of the Julian Day usage. I've updated function signatures, doccumentation, and tests to reflect this.

Other updates:
Fixed a bug where minorMoonPhase was returning a major moon phase

    static func minorMoonPhase(phaseFraction: Double) -> MoonPhase {
--      if phaseFraction < 0.23 {
--        .waxingCrescent
--      } else if phaseFraction < 0.48 {
--        .waxingGibbous
--      } else if phaseFraction < 0.73 {
--        .waningGibbous
--      } else if phaseFraction < 0.98 {
--        .waningCrescent
++      if phaseFraction < 0.25 {
++        return .waxingCrescent
++      } else if phaseFraction < 0.50 {
++        return .waxingGibbous
++      } else if phaseFraction < 0.75 {
++        return .waningGibbous
      } else {
--        .newMoon
++        return .waningCrescent
      }
    }

Refactor julianStartAndEndOfDay to take in a Date instead of a Julian Day

-- static func startAndEndOfJulianDay(julianDay: Double) -> (start: Double, end: Double) {
--       let base = floor(julianDay) + (julianDay.truncatingRemainder(dividingBy: 1) < 0.5 ? -1 : 0)
--       let arr = [0.5, 1.4993].map { base + $0 }
--       return (start: arr[0], end: arr[1])
++ static func julianStartAndEndOfDay(date: Date, timeZone: TimeZone = TimeZone.current) -> (start: Double, end: Double) {
++      var calendar = Calendar.current
++      calendar.timeZone = timeZone
++      let startOfDay = calendar.startOfDay(for: date)
++      let endOfDay = calendar.date(byAdding: .day, value: 1, to: startOfDay)
++      let startJulianDay = AstronomicalConstant.julianDay(startOfDay)
++      let endJulianDay = AstronomicalConstant.julianDay(endOfDay!)
++      return (start: startJulianDay, end: endJulianDay)
    }