New design for strap mount.

Make the strap mount a little thinner.
Make two slots to lock the strap in place
Lengthen the mount slightly.
Replace the square center rod with an octagon.

This looks like a much nicer fit. I need to print to see if it is strong
enough.
First attempt failed because top wasn't strong enough.
Add STLs to git.

Jarrad pointed out that github has an STL diff viewer now.
Add the main STLs to version control.
Fix .gitignore to not ignore STLs.
Modify the Makefile to make generating main STLs easier.