printrbot-simple: Add stud/roller for supporting filament

Used on side of Tower, to use wider filament rolls than
can be supported on the rollers alone
printrbot-simple: Add lower tube for NinjaFlex support

Much simpler than the previous lower part, and while
required a lot of tweaking to print (105%, 40% flow),
also works much better
practical3d: Update soft-hard braclet design
Update README