Add support for the company identifier and maintain backwards compatibility

A suggested fix for [issue #6](https://github.com/FastSpring/fastspring-ruby/issues/6).

This adds an option to define a company identifier in the FastSpring class. If the optional parameter is not given, the class behaves in the exact same way as before, maintaining backwards compatibility. More specifically, it will use the `@store_id` as the company identifier, as before.