Bumper

The bumper is now "functional", checked the microswitch fit last night and it seems okay, maybe a little loose.  I think it's worth getting these changes onto master now before it becomes difficult to merge them.  We can always tweak bits later on.

I've also tried to attach everything with attach() so the assembly instructions work, but I've not run the build to check that it all looks okay, so that may need some tweaking.