GzOutput: Serve compressed files if the browser supports it

This class can be used to serve one or more files compressing them if the user browser supports compression.

It can serve one or more files in the same request. If the files are Javascript or CSS, they can be processed first to remove needless comments and white space.

If the browser supports compression, the class may compress the files data before serving to the browser.

The resulting data may be stored in a cached directory to avoid processing overhead in subsequent requests of the same data.

The class checks the HTTP request headers to avoid serving data that did not change if the browser has cached the data and it was not changed since last time it was served.