Add SOD-523

SOD-523 diode footprint, http://www.diodes.com/datasheets/ap02001.pdf p.144
Anode is pin 1, cathode pin 2, as in KiCAD-included library diodes.
Cathode is marked on silkscreen because of common convention, even thought this is against https://github.com/KiCad/kicad-library/blob/master/KiCad_Library_Convention.txt. 
Diode outline is drawn on DwgsUser layer for reference, not on silkscreen to avoid having silkscreen on pads.