Plugin Endpoint: return next autoupdate time

Include the next scheduled time for autoupdating in our plugin object.
Plugin Modify Endpoint: trigger autoupdate

When someone turns autoupdates on for a plugin, or a group of plugins, we will alter the wp_cron schedule
to run the next autoupdate five seconds after the request.
Plugins Endpoint: custom autoupdate event

Creating a custom trigger to schedule a single autoupdate rather than messing with WordPress built in cron job.
Also, i moved the trigger function to our base class so that we can easily use it from the themes endpoint.
Plugin Endpoint: update the trigger time
Plugin Endpoint: Rethink autoupdates

Yesterday, I attempted to fix it so that when a calypso user chose to autoupdate a plugin,
that we would schedule a cron event to trigger the next autoupdate immediately. Today,
I thought of a better way. Now we will pass an action='update' parameter with the request
anytime we submit a request with autoupdate='true' - this will envoke the ::update() method
in our class, and we won't need to use cron at all.
Plugin Endpoint: fixing a php error
whitespace fix
Merge pull request #1330 from Automattic/update/trigger-plugin-autoupdates

Update/trigger plugin autoupdates