Implemented a new hook point, called hook_boost_preprocess(), for preprocessing (e.g. compressing) the HTML before it is saved to disk.

#242145: Implemented a new setting for configuring whether to purge expired cache files on cron runs or not.

#323395: Allow configuring the full path to the instance-specific cache directory to alleviate HTTP_HOST/SERVER_NAME confusion.

#311988: Prevent caching any HTTP responses other than those of type 'text/html' and status '200 OK'.

Implemented a Drupal sidebar block that tells administrators about the caching/expiration status of the current page.

Got rid of the embedded user ID in the cache directory name. It only annoyed people, and as long as the module only supports anonymous users it is simply of no use. In the future, avoid premature feature creep.

Took over the 'Clear cached data' button on the system performance settings form, making it also clear the static page cache.