Quick-release spool arm

Modified spool support arm for Reprappro Prusa Mendel. No fiddling with nuts or undoing screws, just lift the spool out!

Update 20/7/2013: I have added an extended version of the arm (spool_arm_v2), which is 20mm longer.

I really liked bsthomps' spool holder, although it's for an earlier version of the reprap pro than mine, and you have to unscrew things to remove the spool. I've tweaked the design so that you can just lift the spool axle out of the top of the arm, and I've strengthened t