Autosave:
 * Remove editor.js as a dependency for autosave.js, as it was in 3.5.
 * Remove heartbeat.js as an implicit dependency.
 * Abstract out the serialization of title/content/excerpt for comparisons.

props azaozz.
fixes #24756.



git-svn-id: https://core.svn.wordpress.org/trunk@24774 1a063a9b-81f0-0310-95a4-ce76da25c4cd