Parametric Z-probe endstop holder

I tweaked zennmaster's OpenSCAD file to allow one to change the length of the piece and depth of the channel.

I tweaked this since I was switching to the E3DV6 from the hexagon hotend, the former being slightly longer. Everything else is the same as zennmaster describes in his ABL blogs/videos:

http://zennmaster.com/random-things/auto-bed-leveling-for-the-makerfarm-prusa-i3-part-1-assembly-and-basic-setup

2 variables at the top of the script:
add
deep

The 'add' variable adds to the