Gets the number of available keys in this object.
The keys themselves may be retrieved using key().
length()
// Create the SUP Storage
var store = new hwc.SUPStorage ("one");
store.setItem ("foo", "bar"); // add an item.
store.setItem ("foo1", "bar"); // add an item.
store.setItem ("foo2", "bar"); // add an item.
var result = store.length; // result = 3