Rework Buffer's 'contents-modified' event

This event now fires whenever the content of the buffer changes (after 
a rate-limiting delay) with a single boolean indicating the modified
status of the buffer. There's now a separate event called
'modified-status-changed' to indicate events that change the boolean
value of the isModified method, so we don't need to fire
'contents-modified' when the underlying file is deleted for instance.

src/packages/status-bar/lib/status-bar-view.coffee

@@ -47,7 +47,7 @@ class StatusBarView extends View
   subscribeToBuffer: ->
     @buffer?.off '.status-bar'
     @buffer = @editor.getBuffer()
-    @buffer.on 'contents-modified.status-bar', (e) => @updateBufferHasModifiedText(e.differsFromDisk)
+    @buffer.on 'modified-status-changed.status-bar', (isModified) => @updateBufferHasModifiedText(isModified)
     @buffer.on 'saved.status-bar', => @updateStatusBar()
     @updateStatusBar()
 
@@ -60,8 +60,8 @@ class StatusBarView extends View
   updateGrammarText: ->
     @grammarName.text(@editor.getGrammar().name)
 
-  updateBufferHasModifiedText: (differsFromDisk)->
-    if differsFromDisk
+  updateBufferHasModifiedText: (isModified)->
+    if isModified
       @bufferModified.text('*') unless @isModified
       @isModified = true
     else