Fix slab#181

In firefox 31 with two consecutive newlines it would remove the first
div but the 2nd break on backspace (with ids added for clarity):

<div id='line-1'><br id='br-1'></div>
<div id='line-2'><br id='br-2'></div>

would result in:

<div id='line-1'><br id='br-2'></div>

Line 1 would incorrectly not rebuild() since the outerHTML did not
change. In FF31 a node can have a non-null parentNode even if the node
has been removed from the DOM. Thus our check if a rebuild is necessary
became insufficient. We now check all leaf nodes are descendants of the
line node, not just a parentNode exists.

src/dom.coffee

@@ -218,6 +218,13 @@ DOM =
           DOM.removeNode(curNode.nextSibling)
       curNode = nextNode
 
+  isAncestor: (ancestor, node, inclusive = false) ->
+    return inclusive if ancestor == node
+    while node
+      return true if node == ancestor
+      node = node.parentNode
+    return false
+
   isIE: (maxVersion) ->
     version = document.documentMode
     return version and maxVersion >= version

src/line.coffee

@@ -134,8 +134,8 @@ class Line extends LinkedList.Node
 
   rebuild: (force = false) ->
     if !force and @outerHTML? and @outerHTML == @node.outerHTML
-      if _.all(@leaves.toArray(), (leaf) ->
-        return leaf.node.parentNode?
+      if _.all(@leaves.toArray(), (leaf) =>
+        return DOM.isAncestor(@node, leaf.node)
       )
         return false
     @node = Normalizer.normalizeNode(@node)