Add ANX support as a module

Hi, I am Jonathan, developer from ANX (https://anxbtc.com, https://anxpro.com). Our API is MtGox API v2 compatible therefore I have copied the old mtgox module and make the necessary changes. Test cases are also included. Please accept the pull request. Thanks.

For the detail of our api documentation, please refer to http://docs.anxv2.apiary.io