Ultimate Power Supply Terminal Cover rev2

The original design http://www.thingiverse.com/thing:60392 was great when I first saw it. It did not fit on my 12V power supply correctly, so I revised the design and then added a few extra items to work with my larger switch.

- Model is now more parametric, it scales properly with base_depth
- Slot retainers scale with changes to base_depth
- Switch hole can now be specified on top, or on front of cover
- added output power holes for 12 gauge XT-60 connectors
- Width adjusted to bet