Bug 936645 - [Messages][Drafts] Add banner message for saved draft

- `index.html`
    - Added markup for banner based on [http://buildingfirefoxos.com/building-blocks/status.html](http://buildingfirefoxos.com/building-blocks/status.html)
- `message_manager.js`
    - Add `autoSave` flag on draft auto save when app hides
- `thread_list_ui.js`
    - Added a draft saved duration constant for 5 seconds
    - Added a pointer to the draft saved banner markup
    - Added a draft saved function that shows the banner when called
- `thread_ui.js`
    - Calls `ThreadListUI.onDraftSaved` after a draft is saved, only if `autoSave` is false
- `sms.en-US.properties`
    - Added locale string for banner => "Message saved as draft"
- Tests
    - Unit
        - `message_manager_test.js`
            - Check that autoSave:true is used for auto save operations
        - `thread_list_ui_test.js`
            - Check that banner is hidden, shown for 5 seconds, and hidden again during `onDraftSaved()` if `autoSave` is false
        - `thread_ui_test.js`
            - Check that `onDraftSaved()` is called after draft is saved
    - Manual
        - Save a draft, look for banner
        - Replace a draft, look for banner