rewrite resource loading framework

 * removed chem.onReady(fn)
 * resources.onReady(fn) moved to resources.on('ready', fn)
 * resources is a normal EventEmitter just like everything else
 * added resources.on('progress', fn)
 * resources.bootstrap() no longer called automatically
 * added resources.getTextFile(name, cb)
 * added engine.showLoadProgressBar()
add support for prefetching images

 * rename resources.getTextFile to resources.fetchTextFile
 * add resources.fetchImage
update documentation
Release 0.6.0