Basic auto cron support

This branch adds partial "auto cron" support (issue #1233) to YOURLS in the form of a pluggable action called `cron`.

Every time a page is loaded, check to see if the last `cron` run happened more than 60 seconds ago. If yes, then generate a new HTTP request for `yourls-cron.php` and send it asynchronously via `fsockopen()`. Or alternately, `yourls-cron.php` can be executed by a system cron job via the PHP command line interpreter.

A plugin with complicated scheduling requirements may opt to hook this cron action directly. 

**What's next**
Allow simple plugins to request a periodic action like this.

    yourls_schedule_action( YOURLS_HOURLY, 'myplugin_cleanup' );