Change cancel to off for consistency with emissary

spec/pane-container-spec.coffee

@@ -84,7 +84,7 @@ describe "PaneContainer", ->
       expect(panes).toEqual [pane4]
 
       panes = []
-      subscription.cancel()
+      subscription.off()
       pane4.splitDown()
       expect(panes).toEqual []
 

src/pane-container.coffee

@@ -147,7 +147,7 @@ class PaneContainer extends View
     callback(pane) for pane in @getPanes()
     paneAttached = (e) -> callback($(e.target).view())
     @on 'pane:attached', paneAttached
-    cancel: => @off 'pane:attached', paneAttached
+    off: => @off 'pane:attached', paneAttached
 
   getFocusedPane: ->
     @find('.pane:has(:focus)').view()