open shallow links in window.top.location to avoid iframe errors

If the Pin It button is shown in an iframe, the shallow link to /pin/create/button would try to open inside the iframe and fail due to our X-Frame-Options = SAMEORIGIN header. Failure would interrupt the timing loop that tries to auto-dismiss the Cannot Open Page error, and leave it hanging there.

Fix: open shallow links to /pin/create/button by setting window.top.location, not window.location.

Note: this still does not work on @andymantell's codepen:

    Unsafe JavaScript attempt to initiate navigation for frame with URL 
    'http://codepen.io/anon/pen/lFxnq' from frame with URL   
    'http://s.codepen.io/boomerang/2480149f25bc4a2b7a77ec398740fb351394143224877/index.html'. The 
    frame attempting navigation of the top-level window is sandboxed, but the 'allow-top-navigation' 
    flag is not set.

... but should be fine in situations where the site operator can set `allow-top-navigation`

Thank you: @andymantell and @amurdoch84