Identity acceptance tests

This fixes one of the build errors that crept into the acceptance tests:

```
ashl6947@MMN1F3FD58 ~/code/gophercloud (v0.2.0=) $ script/acceptancetest
# github.com/rackspace/gophercloud/acceptance/openstack/identity/v2
acceptance/openstack/identity/v2/token_test.go:17: cannot use ao (type gophercloud.AuthOptions) as type "github.com/rackspace/gophercloud/openstack/identity/v2/tokens".AuthOptionsBuilder in argument to "github.com/rackspace/gophercloud/openstack/identit:
    gophercloud.AuthOptions does not implement "github.com/rackspace/gophercloud/openstack/identity/v2/tokens".AuthOptionsBuilder (missing ToTokenCreateMap method)
```

Part of #241.