BeagleBone Black slim case - OSHW remix

####Features
This is a remix of Sluttlebot's amazing, parametric [Beaglebone Black Slim Case](http://www.thingiverse.com/thing:471661). My additions are:

- **Larger gap between board and case**
Increased the clearance between board and case from **0.25mm** to **0.5mm**.
- **Wall mounting tabs**
Set an option to select between 2, 4, or no wall mounting tabs. Tabs have holes for #8 screws by default -- the diameter can be modified by changing the value of `Screw_Hole_D` in the *BBB_OSHW