minor update manual
fix vehicle demo (heightfield is broken, when using very large scaling factor)
add btConeShape serialization, see Issue 725
add btGearConstraint get/set, see Issue 685 and Issue 671

git-svn-id: http://bullet.googlecode.com/svn@2665 08e121b0-ae19-0410-a57b-3be3395fd4fd