Redirect limit

Added a redirect limit. I was trying to download a Dropbox URL and it kept hanging, so I thought it was stuck in a re-direct loop, but it looks like that's fixed in https://github.com/mitchellh/vagrant/pull/1497

Anyways, here's an implementation of the redirect limit. I tried adding tests with `webmock` for the redirect itself, but I couldn't find the hooks to get the `download!` method to actually call properly, and it looks like the ingegration tests for it are still pending (https://github.com/mitchellh/vagrant/commit/87bc6ec63f6aedf267b19de219d101c6d254f630#L2R51) so I didn't in the end.