Add pruning to WP_Logging

WP_Logging would now provide a hook to hit with wp-cron so that you could prune old logs.

By default you'd need to set up the cron job and specifically filter the `$should_we_prune` variable (making it true) to make the prune process run.

The default settings are to prune all logs that are 2 weeks old or older and force delete them so that they bypass the WordPress trash.
Merge pull request #8 from curtismchale/patch-2

Add pruning to WP_Logging