Optimise Expires section and Compression

I've merged the "Compression" and "Expires headers" sections from [html5boilerplate's .htaccess](https://github.com/h5bp/html5-boilerplate/blob/master/.htaccess) into selfoss/.htaccess

Compression was not working on the earlier version on a few servers, notably Hostgator's shared server setup. Web fonts also had a relatively short expiration time. The html5boilerplate code resolves both these issues.