modified gmail.rb importer to allow :scope args if provided.

We wanted to make fewer requests for information when authenticating an app to Gmail. By allowing a client app to pass in different scope arguments this is now possible while retaining all the previous default functionality, i.e. if no scope args are passed, the original default scope is used. This was modeled on the preexisting :max_results argument in the initialize method. Specs were provided to validate this functionality.