Fan grill (Customizer)

This thing inspired by [120mm Fan grill - RepRap style by capitaenz](http://www.thingiverse.com/thing:26396).
I made it a symmetric and customizable.

##Customizable parameters are:

- External dimension and mounting holes distance.
- Mounting holes shape and size (cylindrical or for conic head screw)
- Number of "rings" in grill.
- Wide of "rings" and rods.
- Shape of "rings" (true ring or tear drop).
- "Rings" smooth or segmented.
- Central "ring" solid or has hole.

All .stl examples are for 120 mm fan.