Added proper screw cap values

Fixed the TODO: proper screw cap values
Based on: http://www.roymech.co.uk/Useful_Tables/Screws/cap_screws.htm
Diameter is pulled from the table METRIC_BOLT_CAP_DIAMETERS and the cap height is the metric size of the bold.
Update standard_parts.scad

Fixed: "TODO (from MCAD): proper screw cap values: instead of "height" will use METRIC_BOLT_CAP_HEIGHTS[size]+tolerance" this is not necessary because the cap height is the bold size so M3 cap is 3mm heigh.
Also fixed some tolerances because they where terrible big. On my print a M3 hole could fit a M4 bold and in the nut holder the nut could almost turn around. 
Removed radius from hole_for_screw and hole_for_nut to clean up the code and it fixed errors in the STL output of OpenSCAD.
Fixed bolt size issue

Bolt size of the pcb holder could not be chanced. Now it can
StepperMotor mount fix

StepperMotor mount dint render
Merge pull request #23 from D-Byte/patch-1

Patch 1