parametric header pins

Breakaway Header Pins: pitch 2.54, adjustable
Aaron Ciuffo
December 23 2015
GPL V3

derived from Molex KK 100 break-away header:
http://www.molex.com/pdm_docs/sd/022032041_sd.pdf

Usage (default values shown):
```
use
//full
headerPins(columns = 3, rows = 2, centerV = false, center = true, pitch = 2.52);
//simple
headerPins(3, 2);
```
columns = integer number of columns (X axis)
rows = integer number of rows (Y axis)
centerV = vertically center around base - fale places bottom at origin
center = center the array at the origin
pitch = distance between center of each pin

####Note:
* the breakaway connector is a bit of a fudge and does not appear at the same
Z height for centered/versus non centered; this should be largely irrelevant

Updates can be found at GitHub
https://github.com/txoof/OpenSCAD_electronics