Support certs/ that live within $GOPATH/src

This adds an additional search directory that supports calling
git repositories whose certs/ directory is not at $GOPATH/certs, but
rather at $GOPATH/pkg/path/certs. Subpackages such as pkg/path/server
work as well.
I might be wrong on the use of Dir here, but this should still work even if you start with a trailing slash:

http://play.golang.org/p/J9Hv9i7T1o

also you shouldn't need to TrimRight since Dir is going to return the base directory without a trailing slash, and even if it did, filepath.Join handles that for you (see above play example).