use `godef -i` instead of creating a tempfile

godef can only find references in the same pkg if you set `-f` to the
filename and use `-i` to pass the source over standard input. Documented
here: http://godoc.org/code.google.com/p/rog-go/exp/cmd/godef
Merge pull request #21 from wadey/standard-input

use `godef -i` instead of creating a tempfile