Merge pull request #86 from inhies/f.gzip

Improve content type detection

Thanks, @inhies
Minor gofmt from merge
Utilize direct writes rather than buffering

This commit should reduce server load somewhat, because it replaces
bytes.Buffer with direct writes to the http.ResponseWriter.

Close #81
Fix cloning URLs for tree.html and file.html
Fix git.RefExists() to accept branch names

Close #68
Add support for log severity levels