Add link to circleci settings

[RichardLitt]

When a repository does not have circleci enabled, we notice and set a different badge using the image onError feature. We can also link directly to the circleci settings, where you can enable it easiest. For travis, this is already done, too; this is as close as we can get, I think, to automatically enabling from clicking on the badge, and I think all we need, as this is a one-off for each repository (the number of which is, in fact, finite).

Closes #62

[harlantwood]

looks good! did you try disabling circle CI in your hosts, and verify the link works?

[RichardLitt]

I didn't try that, but functionality it shouldn't be any different from a never-enabled CircleCI setting.