Modernization and bugfixes

Original library doesn't seem to be maintained anymore, but here are my changes anyway as we use this fork in production.

- internal zlib (from fork by @cweider) for dynamic content
- internal zlib for staticGzip
- uses modern 'send' npm module to send static data
- fixes race condition in case multiple node instances access same files
- re-enabled HEAD request and it should now work properly as lib uses 'send' module