add support for combined status
fix return type of PullRequests.ListCommits

this method actually returns RepositoryCommits (go-github's term for
this, not GitHub's) rather than Commits.  They are very, very close in
structure (which is likely why this has gone unnoticed for so long), one
of the primary differences being that RepositoryCommits exposes the
GitHub identity of the commit author and committer if possible.
Use https for godoc.org URLs