Implemented getZoneByXY

Implemented `getZoneByXY` in Zone.rb

Additionally, convenience methods for:

*Geohex.encode(latitude, longitude, level)*

    Geohex::V3::Zone.encode(latitude, longitude, level)

*Geohex.decode(code)*

    Geohex::V3::Zone.decode(code)

*Geohex.getZoneByXY(x, y, level)*

    Geohex::V3::Zone.getZoneByXY(x, y, level)