WIP: Split up gutter into multiple components

This is very very much a WIP / experiment. 

This is all it does right now. I have git diff displaying another gutter. 

![screen shot 2014-05-30 at 5 24 25 pm](https://cloud.githubusercontent.com/assets/69169/3137683/00bef802-e85a-11e3-904b-da47b44f9fc2.png)

This is what my git-diff looks like.
```coffee
{$, GutterComponent} = require 'atom'

module.exports =
class ReactGitDiffView
  constructor: (@editorView) ->
    editorView.registerComponent (options) ->
      GutterComponent $.extend {ref: 'gitGutter'}, options
```

The idea is that git diff will use it's own gutter component. minimap could use this mechanism, the fold + bookmarks icon, and any other gutter decorations could use one as well. 

@probablycorey check it. @nathansobo, I'm probably crazy.