Use class variable `auth_token` in `send`.

The `send` method refers to `auth_token` as a local variable instead
of a class variable, and causes the following error:

    global name 'auth_token' is not defined
Merge pull request #1 from jwatson/02ff2b5b3f00b09253dba51da20f85fb45bd615b

Use class variable `auth_token` in `send` method.