Customizable solidoodle alternative snap spool holder

> Companion piece (added 7/10): [Moderately-easy enclosure for Soldoodle](http://www.thingiverse.com/thing:115998).

I really liked the spool holder by nlancaster, but (i) it was a bit tough to print nicely (it was my third print ever, though :); and (ii) the newer Solidoodle spools seem to have larger diameter.

Modified snap-in spool holder, that:
1. Is fully parametric! You can change mounting point, arm length, angle, clip diameter and orientation, etc.
2. Is much simpler to print (but perhaps not as pretty)
3. Adds screw counter-sink
4. Includes full SCAD source (could use some cleanup)

The outer bracket and tower could be one piece, except it's too large for the SD2 .

ChangeLog:
- 7/14: Added STLs I'm using with [companion enclosure](http://www.thingiverse.com/thing:115998).
- 6/15: Added insets for rivets, when choosing back positioning (measured and tested on my SD2, there is some extra slack, let me know in comments if positions are off).
- 5/23: Fixed