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';

    //Set the key 'foo' in localStorage
    storage.local.set('foo', foo);

   //get the key bar  from sessionStorage
    storage.session.get(foo);

});
```