Onboarding tooltip for new editor

Fixes https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/165

When a user opens the new editor the for the first time, a simple tooltip is displayed underneath the "Edit" ```UIBarButtonItem```. Here is a demonstration of how it looks:

https://cloudup.com/cuzDJhL6sNP

The tooltip will go away after 3 seconds or the user can dismiss it by tapping it. After the tooltip is displayed once, it will not be shown again.

/cc @diegoreymendez 
I keep writing constants like you're doing here, so I'm the first offender heh... but [check this out](https://github.com/wordpress-mobile/WordPress-iOS/wiki/WordPress-for-iOS-Objective-C-Style-Guide#constant-definitions).