PrintrBot Spool Holder revision

This is a variant on emmett's PrintrBot Spool Holder, which I really like a lot. It's the best spool holder I've seen, as integrates onto the printer securely without dangling the spool off the side, which can interfere with high-Z builds. I used it for several months until it broke when my PrintrBot fell over during transport in my car. Before printing a new one, I made these mods:

- rounded head
- adjustable guide arm height
- reinforced guide elbow

**UPDATE 13 December