Adds support for TinyMCE 4.x.x (Removes support from 3.x.x)

- Pull upstream (nytimes) master
- Remove TinyMCE 3.5 related code
- Changed folder structure of plugins to match that of TinyMCE 4.x.x

Only thing left (to my knowledge)
- When building, the plugin files are named plugin.js instead of plugin.min.js (any help on that would be awesome)