Update ssc2nd.js

Some code was modified to add Shift support (up to 4K+). So far,
the modifications are a little bit convoluted, requiring that
separate Shift modes be relegated to different stepstypes (2K+ and
4K+ are relegated to dance-routine and dance-couple, respectively),
but the code itself was tested without bugs, using some of my note
charts as test subjects.
Merge pull request #1 from A-zu-ra/master

Adding basic Shift support + some small changes to difficulty detection