Adopt ManageWP's prior art for premium plugins to report when there is an update.

If there's a matching premium plugin with update, it should take priority over WordPress.org
Hook our API on late init, so other plugins can register their actions / filters, CPTs are loaded, etc.
First pass at tricking `Plugin_Upgrader::upgrade()` to accept our mischevious ZIP files :)
Abstract this a bit so we can support updating plugins from arbitrary zips
Merge pull request #86 from humanmade/implement-managewp-api-86

Implement ManageWP's update mechanism for premium plugins / themes