Moving Platforms!

Moved the javascript to its own .js file to keep things a little neater.

Optimized the Platform object
-- Each platform now calls the same set of methods rather than
reloading them into every instance. (Thanks, Norguard @ StackOverflow)

Moving platforms!
-- Inherit most of their code from the Platform object
-- Horizontal & vertical motion supported
-- Still need to make horizontal platforms pull the player along rather
than sliding out from underneath them.