Add support for gzip compression level

I wanted to set the compression level to the maximum, based on the options you can [pass to zlib](http://nodejs.org/api/zlib.html) and [this](http://stackoverflow.com/questions/12467584/how-to-use-node-js-zlib-module-with-options).
Also fixed a typo on the readme. Maybe it would be good to give users the possibility to pass all sort of zlib options, but I think that the compression level is good for most users.
Also gave an example of how not to compress images, because compressing images [is bad](http://developer.yahoo.com/performance/rules.html#gzip). Maybe it would be good to add better docs for the image compression thing in the future.