Support for gzipping files and arbitrary metadata

Sorry for putting these together, but I've added two improvements that I think others will find valuable:

1) runs are now gziped. This saves a lot of space and it I've made it all backward compatible, so any non-gziped runs are handled just as before.

2) You can now optionally include arbitrary metadata with a run. I've updated the documentation with an example. This allows user to include things like $_SERVER['REMOTE_ADDRESS'], the called URI, or anything else. The output is a bit spartan, but's completely optional.