Add vietnamese holidays

[kevinb1989]

Holidays in Vietnam According to Wikipedia:

• New Year's Day: 01-01
• The Day Before Lunar New Year's Eve: 12-29 in Chinese Calendar
• Lunar New Year's Eve: 12-30 in Chinese Calendar
• Lunar New Year Day 1: 01-01 in Chinese Calendar
• Lunar New Year Day 2: 01-02 in Chinese Calendar
• Lunar New Year Day 3: 01-03 in Chinese Calendar
• Lunar New Year Day 4: 01-04 in Chinese Calendar
• Lunar New Year Day 5: 01-05 in Chinese Calendar
• Hung Kings' Festival: 03-10 in Chinese Calendar
• Reunification Day: 04-30
• Labour Day: 05-01
• Independence Day: 09-02
• The day before OR after Independence Day

I copied and refactored the chineseCalendar method from PR #51 to convert dates from Chinese Calendar to Gregorian one. That becomes the ChineseCalendar trait. We might need that trait for the following countries:

• China
• Korea
• Malaysia (PR created)
• Singapore (PR created)
• Vietnam (PR created)

[kevinb1989]

PR looks good, thanks! The Trait is usable for other Countries (like Malaysia) too, I assume?

@Nielsvanpach Thanks! And yes, that can be used by any other Asian countries that also use Chinese Calendars.

[Nielsvanpach]

Thanks!