Raspberry Pi B+ / Pi 2 clam shell case

Another Raspberry Pi B+ / Pi 2 case. The differences to all those _great cases out there on Thingiverse_ are:

- Self centering clam-shell design: Light will not shine though the gaps, robust enclosure
- No lateral space limitation for heat sinks
- Additional round 5mm hole for some GPIO cables
- Combines features of other cases:
- The holes are well fit within an intentional tolerance of 0.5mm
- Well tightened by common 10mm M3 screws and nuts
- By using longer screws case can be st