Added support for a brazilian exchange trader MercadoBitcoin

[arianpasquali]

I altered the code to add support for a new trader that is not covered by bitcoincharts API.
I'm not a ruby expert so it may not be the best implementation, but I wanted to share it anyway.

best

[jawerty]

Thank you for the contribution. However, isn't mercado bitcoin a smaller exchange? Your documenting it like its separate from the other exchanges, like a major market. If I merge, do you mind if I change the code to make it less individualistic to the other code and markets so it doesn't seem like btcl is partial to mercado bitcoin?

[arianpasquali]

Indeed is a smaller exchange compared to the others, but it is maybe the most active in Brazil. I don't mind at all if you change or adapt the code to be more generic. You are right. It seams partial the way it is.