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 2015: ** New changes in version "tkb2":
- variable arm thickness
- improved arm filament guide shape using sphere hulls
- added various 1/4" mounting holes at base and on shaft
- guide elbow converted to nice sweeping curve instead of cylinder