Parametric pulley - as a module

Remixed *the original* to able to use pulley generator in code like that:

```
use

pulley ( "T2.5" , 25 , 0.7 , 1.678 );
```

This enables us to i.e. translate the thing in the openscad easilly.

*NOTE*:
_ original build with include is still available, use `use` for mixing in other openscad files_


In next steps I'll move the two magic numbers out of function too and put on github for easier update.