Add locale for en-HK

[lauft]

The following locale needs to be verified/completed:

class en_HK(Locale):
    """
    01-01: [NF] The first day of January
    05-01: [NF] Labour Day
    07-01: [NF] Hong Kong Special Administrative Region Establishment Day
    10-01: [NF] National Day
    12-25: [NRF] Christmas Day
    12-26: [NRF] The first weekday after Christmas Day
    : [NV] Lunar New Year’s Day
    : [NV] The second day of Lunar New Year
    : [NV] The third day of Lunar New Year
    2 days before Easter: [NRV] Good Friday
    : [NV] The day following Good Friday
    : [NRV] Ching Ming Festival
    : [NV] The Birthday of the Buddha
    : [NV] Tuen Ng Festival
    : [NV] The day following the Chinese Mid-Autumn Festival
    : [NV] Chung Yeung Festival
    """

    locale = "en-HK"
    easter_type = EASTER_WESTERN

Checklist:

• Are all holidays (i.e. days "which are defined by law on which business or work are suspended or reduced") listed? Can you provide the legal sources which define the holidays?
• Is the spelling correct?
• Are the fixed dates <month>-<day> correct?
• If a holiday has a variable date: Is the algorithm to calculate it correct/can you provide it if missing? If it depends on the easter date: Is it the western or orthodox easter date?
• If a holiday is proclaimed by the government: Can you provide the source?
• Are holidays moved to a different date, e.g. if they fall on a saturday/sunday/...?
• Are holidays observed on a different date (is there a substitute holiday), e.g. if they fall on a saturday/sunday/...?
• Are the flags correct? (N = national holiday, defined by law, R = holiday with religious background, F = holiday with fixed date, V = holiday with variable date)
• Have there been any holidays renamed/deleted/created since 2011? If yes, which ones and since when?