In buffer change event objects, rename 'string' to 'newText'

In preparation for adding an 'oldText' value.

spec/atom/buffer-spec.coffee

@@ -116,7 +116,7 @@ describe 'Buffer', ->
       expect(changeHandler).toHaveBeenCalled()
 
       [event] = changeHandler.argsForCall[0]
-      expect(event.string).toBe newText
+      expect(event.newText).toBe newText
       expect(event.oldRange).toEqual expectedPreRange
       expect(event.newRange).toEqual(new Range([0, 0], [1, 14]))
 

src/atom/buffer.coffee

@@ -83,7 +83,7 @@ class Buffer
       newTextLines[lastLineIndex] += suffix
 
     @lines[oldRange.start.row..oldRange.end.row] = newTextLines
-    @trigger 'change', { oldRange, newRange, string: newText }
+    @trigger 'change', { oldRange, newRange, newText }
 
   save: ->
     if not @path then throw new Error("Tried to save buffer with no url")