0.4.0 Release

This PR adds the following functionality to Sir Trevor; almost certainly breaking some backwards functionality between this and `0.3.x`.

* Adds a mediator for each Editor instance 
* Moves all block handling code from the Editor into the `BlockManager`
* Renames many of the methods on the `BlockStore` (as per #155)
* Adds a dedicated `ErrorHandler` 
* Standardize naming conventions of files 
* Clean up the `EditorStore`

cc @cobbweb @ninjabiscuit @ilyaporopudas @andrewsprinz