The startup script now uses a `Set` to manage `GitDiffView`s held in memory and destroy them when `deactivate` is called.
There are now four major subscription blocks.
1. The outer subscriptions held by `activate`.
2. The per-editor subscriptions held within `activate`.
3. The per-editor repository event subscriptions held within each `GitDIffView` instance.
4. The per-editor modification event subscriptions held within each `GitDiffView` are only active when the editor content is bound to a valid git repository.
Teardowns of any editor or the module now result in `disposal` of the respective editor's subscriptions or all subscriptions authored within the module.
I removed some of `GitDiffView`'s unnecessary methods such as the `start`, `cancleUpdate`, `addDecoration` and `removeDecorations`;
The last two methods were combined into the body of `updateDiffs`.
`scheduleUpdate` now calls `requestAnimationFrame` instead of `setImmediate` because it's native, standard, and yields
to other more important browser processes. I know Atom Core implements setImmediate, but rAF seems to work just as fast if not faster.
The memory management of the editor markers and diffs have been joined using a WeakMap. When the diffs are destroyed,
so too are the editor markers.
Finally, I added the `destroy` method to handle the teardown of subscriptions and other destroyable objects contained within the `GitDiffViews` before object release.
