Fix getGitHubDir failing sometimes because it didn't respect GitHub API pagination.

Fixes #174.

- If the response contains a Link header, then fallback to requesting "refs/heads/master" and "refs/tags/go1" by name.
- Add hasLinkHeader return value to getJSON(), returns true if `Link` header was present.

Note that it currently assumes "master" has to be a branch, and "go1" has to be a tag. I don't know what's the best thing to do (and what if "master" is both a branch _and_ a tag, for example?). Please see discussion [here](https://github.com/golang/gddo/issues/174#issuecomment-47871865).
Yeah, that's the recommended [Go style](https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Initialisms). Fixed in 60182583d83c2354487625d9d019961c5d827972.

Though I personally don't follow it, because such scheme is lossy. I.e., you can't break HTMLJSONURL into words without human intervention, but HtmlJsonUrl is [easy to do so](https://gist.github.com/shurcooL/6003701#file-main-go-L24).