Added support for horizontal scrolling.
Kind of a freebie - the mobile app is already sending x scroll info, and the server code even picks it out when parsing scroll packets. Just a matter of generating corresponding left and right scroll button events.
Made horizontal scrolling optional; off by default
Made [maximum] scroll speed configurable
scrollMax option puts a cap on how many scroll-key events can be issued per scroll motion. Scroll motion events come with pixel offsets, but treating each offset unit as a separate "scroll key" event yields whiplash-fast scrolling. Default scrollMax of 1 may be too conservative for some but feels usable to me.