API update. 

Hi there,
I was planning on using your library and found that it was not working with the latest version of the the API.  I see in some of the comments [1] in the issues that you are no longer using/updating the library so I decided to contribute.

I made the following changes:

- Deprecation of legacy API
- Support for v1 and v2 of the API.
- Automatic detection of the API version based on method name. You can use the same instance of the class to call methods from all the API versions.

Cheers.

1. [https://github.com/mndrix/Finance-MtGox/issues/6](https://github.com/mndrix/Finance-MtGox/issues/6)