Remove unused requires
Add TextEditorElement onDidAttach/Detach for space-pen TextEditorView
Export event-kit classes from exports/atom.coffee

CompositeDisposable is needed by TextEditorView, which is now extracted
to the atom-space-pen-views library.
:arrow_up: atom-space-pen-views and enhance deprecated exported views

The atom-space-pen-views library now imports its own version of SpacePen
without a lot of the extensions that we add in the version we export
in Atom. In order to maintain the same functionality in the views we
export, we need to update their prototypes to point at the enhanced
SpacePen views instead.
Avoid adjusting prototypes of SpacePenViews globally

The only constructors that get their prototypes adjusted are the ones
that are exported from exports/atom.coffee.