storage.js - some fixes and improvements

Starting from #83, I would better go for a systematic JSON serialization, so
`game.storage.set('some.key', 42)` would return a `Number` when retrieving it.
I also provided a mean to get a default value if the key is not present:
`game.storage.get('some.key', "default value")`
And an utility `game.storage.has('some.key')`
I also sticked to the `localStorage` API (which is faster and more robust).