Visual editor update for v4.4

These changes were initially committed by @bummytime in [this PR](https://github.com/wordpress-mobile/WordPress-iOS/pull/2313).

This PR contains the new visual editor (WordPress-iOS-Editor v0.2.0). By default, the visual editor **will be disabled** for all users and the existing editor will be used. The reasons for this are twofold:

1) The team feels the editor is very close to completion and should be made available to a select few users, but not quite the entire world yet.
2) We would like to get metrics from real-world usage so we can improve it even more.

To unlock the editor, you will need to use this special URL: `wordpress://editor?available=1&enabled=1` in Safari. Subsequently, you can turn on/off the visual editor by flipping this bit in Settings:

![d91cc9ba-32e5-11e4-813b-0d6093b0e09b](https://cloud.githubusercontent.com/assets/1836005/4135598/8b85afea-3377-11e4-811d-46d5acbd2fea.png)

The existing editor classes are now prefixed with `WPLegacy*` and are identical to the ones that shipped with 4.3.

/cc @diegoreymendez @thianhlu @sendhil @mikejohnstn