Ability to specify the channel to run

The channel feature of Elysia cron is great. This patch adds the ability to run a single channel by calling

http://my_site.com/cron.php?channel=my_channel

This can be useful if you have a site running on multiple servers (behind a load balancer) and you wish to distribute some lengthy task (like content indexing) on each server.

Note that most of the time, cron task are not written to be distributed across multiple servers. You will most likely have to write your own version of the tasks.