Version 0.0.16

* Added gzip compression for static files except images and small files
(256 bytes)
* Optimized HTTP 304 for gzipped static memory cache
* Performance tested and optimized
* Fixed HTTP headers generation: Last-Modified, Content-Type,
Content-Length
* Added method impress.signIn (session creates only if not already
exists)
* Added method impress.signOut (session remains but it unlinks user)
* Changed impress.register, simplified for applied developers
* Use req.impress.logged to ckeck if user logged, not
req.impress.session, there are session allowed with no logged user
* Fixed bug in restoreSession