Implemented remote autosaves

A post will be autosaved:

* Every 20 characters changed or so
* When you stop typing for 5 seconds
* When you tap on preview

Known issues:

* The "Discard/Save" alert when the Cancel button is tapped is shown even if the post is fully autosaved
* If you start a new post and it autosaves, choosing the "Discard" option only discards the changes since the last autosave

git-svn-id: http://ios.svn.wordpress.org/trunk@4023 7d847f7c-ffe1-4a24-85be-b788d016d96a