Mbed Nucleo-64 Breadboard

Improved version of the original, with slightly larger opening for Nucleo board.

Changes I made to original:
- Removed large breadboard holder for smaller size
- Widened Nucleo board cutout so it fits easier (as requested by comments)
- Made the mounting holes smaller so that simple M3 screws can self-tap into the PLA
- Provided a cutout in the side to give better clearance to USB cables inserted into Nucleo