Initial Mt.Gox support

Support for querying the price of Bitcoin/Litecoin/Namecoin (BTC/LTC/NMC) on the Mt.Gox exchange. Data is retrieved via a public API (documented by third parties and widely used) and then parsed with the JSON module.

I added one method for each market that Mt.Gox operates, named after the market's currency code e.g., `mtgox_usd`. These methods are also available for fallback under generic names such as `bitcoin_usd`. I welcome suggestions for a better name for the fallback methods, given that they also retrieve data for LTC/NMC and any other currency that Mt.Gox offers. Perhaps `digital_currency_usd`, what do you think?

https://rt.cpan.org/Public/Bug/Display.html?id=84790