Modernization and bugfixes

[tikonen]

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

[gcpantazis]

@nateps, +1 to merging this. We're using @tikonen's fork as well... resolved our issues with the current NPM build.

[tcbeutler]

@nateps +1 for merging this, also using @tikonen 's fork. Seems to be an issue with socket.io and gzip on Windows, getting "This socket is closed."

socketio/socket.io#932