:bug::shoe: prevents orphaned editor in buffer destroy listener

Previously, the whitespace package would bind listeners on `buffer` events per `editor`, and unbind when the `buffer` was `destroyed`.

The problem is apparent when you use split a `pane` because, currently, the new copied `editor` is assigned the same `buffer`. As a result, if you close one of the `editor`s, the listener is still bound with a reference to an editor that is no longer in your active `workspace` (because the `buffer` hasn't been destroyed yet).

This ultimately throws when you call `getSelectedBufferRanges` on the orphaned `editor` in `ensureSingleTrailingNewline` (introduced in dbb992ba723a25a953c63b55d5edf669ebf637ea).

My test is probably pretty bad (`getActivePane`? :rage4:) but I bounced around dev tools until I learned _just_ enough API to write it so no hard feelings if you :trashcan: it.

Also, make :trashcan: a thing pretty please.

Here's a gif demonstrating the issue:
![buffer](https://f.cloud.github.com/assets/62923/2392545/864335b6-a97a-11e3-8906-807f05933cbf.gif)

Steps to reproduce:
1. Enable `ensureSingleTrailingNewline`
1. Save a file with a trailing new line
2. Split that file
3. Close the original editor
4. Remove trailing newline from the other editor
5. Attempt to save