Modificaton to skadis_bits_serie, add spacing

[ErnieV]

Hi,
I'm not sure how to do a pull request but I'll add the modified file:
ikea_skadis_erwin.scad.zip

It gives the option to do a spacing in both x and y direction between the bits. I need this because I use the bits holder without a bottom as a screwdriver holder. Having the h parameter set to 40 or so makes them hang more straight than in a rack.

Hope you find it useful and thank you for all the effort you put in this!

[ErnieV]

Also, I reduced the retainer a bit because it wouldn't fit for me.
Mayby parameterize that?