Added file for special extras

I thought to include some specials like springs / trampolines that
launch you into air and later on also to implement power ups so I
created a file for that and even wrote a little code for springs
already. Works great by the way
Small bug fix

when you would jump on a platform from underneath it would stop
unnaturally. I fixed that
Added "extra" ("ex") variable to platforms
Added test-springs to the file