Add ability to use sandbox OAuth credentials to authenticate to EBay.

I've made the environment optional, defaulting to production, so that users of this gem can test their code in the sandbox before using it in a production environment.  Note there is a change to the arguments in the omniauth initializer - I didn't think passing the API url was all that useful and should be derived from the environment flag with this change.  AFAIK the EBay XML API doesn't really change.

Tests and docs have been updated in line with code changes, let me know what you think.