Tweaked the logic of parachutes to be more modifiable. There now is an upper bound percentage and lower bound percentage which will be applied if you are driving faster and slower respectively. Also added those values in stk_config.xml to make playtesting easier (ticket #1135)
Implemented the possibility to give yourself "negative" attachments (parachutes, anvils, bombs) to be able to playtest more efficiently
Changed lower and upper bound (at lower speed less speed should be
lost), capped speed fraction. Tweaked parachutes parameters somewhat.