do not override the signup url if the user is an admin or a subscriber; see https://github.com/GigaOM/legacy-pro/issues/4415
turns out we didn't need to check all those user caps. "subscriber" would do.
Merge pull request #33 from wluo/github-4415

do not override the signup url if the user is an admin or a subscriber
Merge branch 'master' of github.com:GigaOM/go-recurly into freebird-v2