OpenSCAD 2D honeycomb shape module

Include in your project by:
`include ;`

Make a honeycomb by:
`honeycomb(r=4, thickness=2, cols=5, rows=4, center=true);`

Module args are:
- r float radius of hexagons
- thickness float thickness of hexagon walls
- cols int how many hexagons per row
- rows int number of rows
- center bool center generated shape

Use `linear_extrude()` to make it 3D.

Update 19/03/2017
Added module `honeycomb_square()` which lets you specify x and y rather than rows and cols so it's easier to fit