Add basic support for creating gzipped sitemap files

Added 4 configuration:
* gzip - if true, will generate on populate and will also display the gz version when accessing sitemap index.
* gzip_dir - directory where to store the gzip files.
* gzip_file_pattern - filename pattern.
* gzip_url - either absolute or relative url where to access the file. It will still use gzip_file_pattern as pattern for the gz files. When a relative url is specified, it will use base_url.