Add accessor for orphaned HEAD.
Clarify documentation on `isHeadDetached`.
Use `git_repository_head` instead of going through GTReference.

This is equivalent, but has a special error code for orphaned HEADs.
Merge branch 'master' into stuff
Add private `-lookupByGitOid:` to GTRepository.
Capitalization.
Change the property name.
Merge pull request #230 from libgit2/stuff

Some changes I gathered while working