Patch indexedDB shim to work with Safari 7.1+

fixes noflo/noflo-ui#404 :bear: 

Poorly-supported idb (Safari 7.1-8.0) still needs the shim, but can’t set `window.indexedDB`; so we need to expose it as `window.overrideIndexedDB`

Papered over that with `vendor/IndexedDB`

Tested with 
iOS 8, 7, 6
Desktop Safari 8.0, 7.1, 7.0, 6.1
 :+1: