Add vietnamese holidays

[kevinb1989]

Holidays in Vietnam:

• New Year's Day: 01-01
• 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
• Hung Kings' Festival: 03-10 in Chinese Calendar
• Reunification Day: 04-30
• Labour Day: 05-01
• Independence Day: 09-02

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
• Vietnam (PR created)