Further abstract storage API

[jjrscott]

Some platforms, notably the iOS simulator, change document.URL on every run, resulting in lost data.

Moving document.URL into storagewrapper.js allows for:

1. Cleaner calls in engine.js and globalVariables.js
2. Simpler overriding as now only the key is included in the call.

This change should have no effect on existing installations since the underlying localStorage keys remain the same.