Move gutter member into a showLineNumbers observe block

When `editor.showLineNumbers` is toggled, the `ReactEditorView.gutter` member becomes stale. This moves it under an observer for `editor.showLineNumbers` so when it is toggled, the `gutter` member is reinstated. 

This is required for atom/git-diff#30. Ideally there would be a way to decorate a node similarly to decorations so we didnt have to do this hackery and related bs in git-diff. Something like `decorateObject('gutter', {class: 'blah'})`.