Simple Coins Factory

The module allows you to easily create your own simple coins.
I've completely refactored the original module to separate code from data. As the result you can easily define parameters for new coins.
The module can be used as is (i.e. without modifications) since it accepts parameters from outside (refer to provided `Simple_coins_test.scad` for examples).
The generated coins should be easily printable because all bridges (produced by the original module) are eliminated: the bottom part is flat and the nominal value is engraved in it.

#### History
2017-Jan-22
* Changed default edge height (`EDGE_H = 1`) to make coins with good relief (as in the original design)
* Introduced global boolean parameter (`EXTRA_EDGE` with default value `false`) to generate (if `true`) extra edge on the top to allow easy stacking