Fix log formatting error caught by `go vet ./...`.

gddo-server/main.go:257: possible formatting directive in Print call.
Make one log.Printf call consistent with all others by not adding \n at
the end (it's a noop, because log package always adds a \n at the end
if it's missing anyway).
Fix getGitHubDir failing sometimes because it didn't respect GitHub API pagination.

Fix #174.

If the response contains a Link header, then fallback to requesting
"refs/heads/master" and "refs/tags/go1" by name.

Use Go style for type naming.
See https://github.com/golang/gddo/pull/178#discussion_r14546920.

Modify to getJSON to return (*http.Response, error).
Check for the link header in GitHub specific code.
Excellent suggestions from
https://github.com/golang/gddo/pull/178#issuecomment-48007781. Thanks
@garyburd!
Merge pull request #178 from shurcooL/fix/github-pagination-issue

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