Jetpack Options: adding a new autoupdate_core option
Core Endpoint: adding a new core endpoint

There is currently an endpoint to update the core version. Adding an endpoint that
will GET core info, and toggle autoupdates.  Moving find_latest_core_version function
into new class so that it is available to both core related classes
Merge branch 'automatic-updates' of https://github.com/Automattic/jetpack into automatic-updates