Provide S3 classes for return objects

[richierocks]

Currently the return value of all API calls is (I think) asana_api.

It could be useful to make use the class to make it clear what type of Asana object the thing is. That is, is it a workspace/project/task/user/section, etc.

For example, the result of asn_tasks_create() should have class c("asana_task", "asana_api"). Likewise the result of asn_projects_create() should have class c("asana_project", "asana_api").