Specs: default exports to {} rather than document, since it makes writing modules saner

sky/specs/apis.md

 APIs
 ====
 
 The Sky core API
 ----------------
 
 ```javascript
 module 'sky:core' {
 
   // EVENTS
@@ skipping 293 matching lines @@
     //     the values passed in on options
     //  6. register the new element
 
     readonly attribute ScriptElement? currentScript; // O(1) // returns the <script> element currently being executed if any, and if it's in this module; else null
   }
 
   class Module : AbstractModule {
     constructor (Application application, Document document, String url); // O(1)
     readonly attribute Application application; // O(1)
 
-    attribute any exports; // O(1) // defaults to the module's document
+    attribute any exports; // O(1) // defaults to {}
   }
 
   class Application : AbstractModule {
     constructor (Document document, String url); // O(1)
     attribute String title; // O(1)
   }
 
   // see script.md for a description of the global object, though note that
   // the sky core module doesn't use it or affect it in any way.
 
@@ skipping 169 matching lines @@
   the core mojo fabric JS API   sky:mojo:fabric:core
   the asyncWait/cancelWait mojo fabric JS API (interface to IPC thread)  sky:mojo:fabric:ipc
   the mojom for the shell, proxying through C++ so that the shell pipe isn't exposed  sky:mojo:shell
   the sky API  sky:core
   the sky debug symbols for private APIs  sky:debug
 ```
 
 TODO(ianh): determine if we want to separate the "this" from the
 Document, especially for Modules, so that exposing a module's element
 doesn't expose the module's exports attribute.