Altered Sugarcane history to use SessionStorage

One problem with storing past history states using
window.history.pushState() is that Firefox places a 640kb limit on the
data passed to it - a limit which could crash a Twine game with many
variables and data structures. This is actually for good reason - web
apps with heavy state should invoke SessionStorage rather than just
pushing it to browser history. So, SessionStorage usage has now been
implemented.