Add support for Australian GST

[CloCkWeRX]

http://en.wikipedia.org/wiki/Goods_and_Services_Tax_(Australia) covers it off, but effectively it's:

• 10% on certain goods and services
• Prices should be always GST inclusive when displayed to a user
• Invoices render the total, gst, and combined total
• 1 July 2000 is the first date it came into effect