sessionStorage

As discussed with @jamesgorrie and @stephanfowler this refactors our ```storage.js``` module to also abstract the sessionStorage api. 

storage.js is now a singleton object which contains two instances of the Storage class, one for session and one for local. The api of the class remains the same. 

### Example
```JavaScript
define(['storage'], function(storage) {

    var foo = 'bar';

    storage.local.set(foo);



});
```