We had skipped some steps in the spec and were:
* Always broadcasting an old value of null, instead of what it
actually was previously.
* Still broadcasting a storage event even if the value had
not changed in storage compared to the last value.
Fix both issues by returning what the old value is in the setter and
implementing the missing logic.
Instead of storing all storage objects in static memory, we now
follow the the spec by lazily creating a unique Storage object
on each document object.
Each Storage object now holds a 'proxy' to the underlying backing
storage. For now, this proxy is simply a reference to the backing
object. In the future, it will need to be some type of interface
object that stores on a SQLite database or similar.
Session storage is now correctly stored / tracked as part of the
TraversableNavigable object.
Local storage is still stored in a static map, but eventually this
should be factored into something that is stored at the user agent
level.
