Add G20 day (Brisbane) and show day (Cairns)

See http://www.justice.qld.gov.au/fair-and-safe-work/industrial-relations/public-holidays/dates for G20, and http://www.justice.qld.gov.au/fair-and-safe-work/industrial-relations/public-holidays/2014-show-holidays for Cairns.

I'm working on a data mining project at the moment, to add all the regional Australian holidays. Expect more pull requests soon :)
Maybe we can add yaml syntax to specify a specific date. That would probably be more performant. I'm not sure if once-off holidays are a big thing worldwide, but we love them in AU.